SQL语言 - 语法基础

全称 Structured Query Language,结构化查询语言。操作关系型数据库的编程语言,定义了一套操作关系型数据库统一标准 。

通用语法

在学习具体的SQL语句之前,先来了解一下SQL语言的同于语法。

  1. SQL语句可以单行或多行书写,以分号结尾。
  2. SQL语句可以使用空格/缩进来增强语句的可读性。
  3. MySQL数据库的SQL语句不区分大小写,关键字建议使用大写。
  4. 注释:
    • 单行注释:-- 注释内容 或 # 注释内容
    • 多行注释:/* 注释内容 */

SQL分类

SQL语句,根据其功能,主要分为四类:DDL、DML、DQL、DCL

分 类 全称 说明
DDL Data Definition Language 数据定义语言,用来定义数据库对象(数据库,表,字段)
DML Data Manipulation Language 数据操作语言,用来对数据库表中的数据进行增删改
DQL Data Query Language 数据查询语言,用来查询数据库中表的记录 ⭐️
DCL Data Control Language 数据控制语言,用来创建数据库用户、控制数据库的访问权限

DDL(数据定义语言)

Data Definition Language,数据定义语言,用来定义数据库对象(数据库,表,字段) 。

数据库操作

查询所有数据库:
SHOW DATABASES;
查询当前数据库:
SELECT DATABASE();
创建数据库:
CREATE DATABASE [ IF NOT EXISTS ] 数据库名 [ DEFAULT CHARSET 字符集] [COLLATE 排序规则 ];
删除数据库:
DROP DATABASE [ IF EXISTS ] 数据库名;
使用数据库:
USE 数据库名;
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实际操作:

# 查询所有数据库
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| performance_schema |
| student            |
| sys                |
+--------------------+
4 rows in set (0.00 sec)

# 测试创建数据库,最好加上 if not exists条件,再去创建
mysql> create database test_db;
Query OK, 1 row affected (0.00 sec)

mysql> create database test_db;
ERROR 1007 (HY000): Can't create database 'test_db'; database exists

mysql> create database if not exists test_db;
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| performance_schema |
| student            |
| sys                |
| test_db            |
+--------------------+
5 rows in set (0.00 sec)

# 测试删除数据库,最好加上 if exists条件,再去删除
mysql> drop database test_db;
Query OK, 0 rows affected (0.06 sec)

mysql> drop database test_db;
ERROR 1008 (HY000): Can't drop database 'test_db'; database doesn't exist

mysql> drop database if exists test_db;
Query OK, 0 rows affected, 1 warning (0.00 sec)


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注意事项:

  • UTF8字符集长度为3字节,有些符号占4字节,所以推荐用utf8mb4字符集

表操作

表操作-查询

查询当前数据库所有表:
SHOW TABLES;
查询表结构:
DESC 表名;
查询指定表的建表语句:
SHOW CREATE TABLE 表名;
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实际操作:

mysql> use student;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| student           |
+-------------------+
1 row in set (0.00 sec)

mysql> desc student;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| id    | int(11)             | NO   | PRI | NULL    |       |
| name  | varchar(30)         | YES  |     | NULL    |       |
| age   | int(11)             | YES  |     | NULL    |       |
| sex   | enum('man','woman') | YES  |     | NULL    |       |
| class | int(10)             | YES  |     | NULL    |       |
+-------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

mysql> show create table student;
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                           |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| student | CREATE TABLE `student` (
  `id` int(11) NOT NULL,
  `name` varchar(30) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `sex` enum('man','woman') DEFAULT NULL,
  `class` int(10) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
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创建表结构:

CREATE TABLE 表名(
	字段1 字段1类型 [COMMENT 字段1注释],
	字段2 字段2类型 [COMMENT 字段2注释],
	字段3 字段3类型 [COMMENT 字段3注释],
	...
	字段n 字段n类型 [COMMENT 字段n注释]
)[ COMMENT 表注释 ];
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注意:最后一个字段后面没有逗号

表操作-案例

设计一张员工信息表,要求如下:

  1. 编号(纯数字)
  2. 员工工号 (字符串类型,长度不超过10位)
  3. 员工姓名(字符串类型,长度不超过10位)
  4. 性别(男/女,存储一个汉字)
  5. 年龄(正常人年龄,不可能存储负数)
  6. 身份证号(二代身份证号均为18位,身份证中有X这样的字符)
  7. 入职时间(取值年月日即可)

建表语句:

create table emp(
	id int comment '编号',
	workno varchar(10) comment '工号',
	name varchar(10) comment '姓名',
	gender char(1) comment '性别',
	age tinyint unsigned comment '年龄',
	idcard char(18) comment '身份证号',
	entrydate date comment '入职时间'
) comment '员工表';
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表操作-修改

添加字段:
ALTER TABLE 表名 ADD 字段名 类型(长度) [COMMENT 注释] [约束];
例:ALTER TABLE emp ADD nickname varchar(20) COMMENT '昵称';

修改数据类型:
ALTER TABLE 表名 MODIFY 字段名 新数据类型(长度);
修改字段名和字段类型:
ALTER TABLE 表名 CHANGE 旧字段名 新字段名 类型(长度) [COMMENT 注释] [约束];
例:将emp表的nickname字段修改为username,类型为varchar(30)
ALTER TABLE emp CHANGE nickname username varchar(30) COMMENT '昵称';

删除字段:
ALTER TABLE 表名 DROP 字段名;

修改表名:
ALTER TABLE 表名 RENAME TO 新表名

删除表:
DROP TABLE [IF EXISTS] 表名;
删除表,并重新创建该表:
TRUNCATE TABLE 表名;
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实际操作

  1. 创建 emp 表
  2. 添加字段: 为emp表增加一个新的字段”昵称”为nickname,类型为varchar(20)
  3. 修改数据类型: nickname,类型为varchar(30)
  4. 修改字段名和字段类型: 将emp表的nickname字段修改为username,类型为varchar(40)
  5. 删除字段: 将emp表的字段username删除
  6. 修改表名: 将emp表的表名修改为 employee
create table emp(
    id int comment '编号',
    workno varchar(10) comment '工号',
    name varchar(10) comment '姓名',
    gender char(1) comment '性别',
    age tinyint unsigned comment '年龄',
    idcard char(18) comment '身份证号',
    entrydate date comment '入职时间'
) comment '员工表';

mysql> show tables;
+-------------------+
| Tables_in_student |
+-------------------+
| emp               |
| student           |
+-------------------+
2 rows in set (0.00 sec)

mysql> ALTER TABLE emp ADD nickname varchar(20) COMMENT '';
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+-----------+---------------------+------+-----+---------+-------+
| Field     | Type                | Null | Key | Default | Extra |
+-----------+---------------------+------+-----+---------+-------+
| id        | int(11)             | YES  |     | NULL    |       |
| workno    | varchar(10)         | YES  |     | NULL    |       |
| name      | varchar(10)         | YES  |     | NULL    |       |
| gender    | char(1)             | YES  |     | NULL    |       |
| age       | tinyint(3) unsigned | YES  |     | NULL    |       |
| idcard    | char(18)            | YES  |     | NULL    |       |
| entrydate | date                | YES  |     | NULL    |       |
| nickname  | varchar(20)         | YES  |     | NULL    |       |
+-----------+---------------------+------+-----+---------+-------+
8 rows in set (0.01 sec)

mysql> ALTER TABLE emp MODIFY nickname varchar(30);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+-----------+---------------------+------+-----+---------+-------+
| Field     | Type                | Null | Key | Default | Extra |
+-----------+---------------------+------+-----+---------+-------+
| id        | int(11)             | YES  |     | NULL    |       |
| workno    | varchar(10)         | YES  |     | NULL    |       |
| name      | varchar(10)         | YES  |     | NULL    |       |
| gender    | char(1)             | YES  |     | NULL    |       |
| age       | tinyint(3) unsigned | YES  |     | NULL    |       |
| idcard    | char(18)            | YES  |     | NULL    |       |
| entrydate | date                | YES  |     | NULL    |       |
| nickname  | varchar(30)         | YES  |     | NULL    |       |
+-----------+---------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)

mysql> ALTER TABLE emp CHANGE nickname username varchar(40) COMMENT '';
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+-----------+---------------------+------+-----+---------+-------+
| Field     | Type                | Null | Key | Default | Extra |
+-----------+---------------------+------+-----+---------+-------+
| id        | int(11)             | YES  |     | NULL    |       |
| workno    | varchar(10)         | YES  |     | NULL    |       |
| name      | varchar(10)         | YES  |     | NULL    |       |
| gender    | char(1)             | YES  |     | NULL    |       |
| age       | tinyint(3) unsigned | YES  |     | NULL    |       |
| idcard    | char(18)            | YES  |     | NULL    |       |
| entrydate | date                | YES  |     | NULL    |       |
| username  | varchar(40)         | YES  |     | NULL    |       |
+-----------+---------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)

mysql> ALTER TABLE emp DROP username;
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc emp;
+-----------+---------------------+------+-----+---------+-------+
| Field     | Type                | Null | Key | Default | Extra |
+-----------+---------------------+------+-----+---------+-------+
| id        | int(11)             | YES  |     | NULL    |       |
| workno    | varchar(10)         | YES  |     | NULL    |       |
| name      | varchar(10)         | YES  |     | NULL    |       |
| gender    | char(1)             | YES  |     | NULL    |       |
| age       | tinyint(3) unsigned | YES  |     | NULL    |       |
| idcard    | char(18)            | YES  |     | NULL    |       |
| entrydate | date                | YES  |     | NULL    |       |
+-----------+---------------------+------+-----+---------+-------+
7 rows in set (0.00 sec)

mysql> ALTER TABLE emp RENAME TO employee;
Query OK, 0 rows affected (0.02 sec)

mysql> desc employee;
+-----------+---------------------+------+-----+---------+-------+
| Field     | Type                | Null | Key | Default | Extra |
+-----------+---------------------+------+-----+---------+-------+
| id        | int(11)             | YES  |     | NULL    |       |
| workno    | varchar(10)         | YES  |     | NULL    |       |
| name      | varchar(10)         | YES  |     | NULL    |       |
| gender    | char(1)             | YES  |     | NULL    |       |
| age       | tinyint(3) unsigned | YES  |     | NULL    |       |
| idcard    | char(18)            | YES  |     | NULL    |       |
| entrydate | date                | YES  |     | NULL    |       |
+-----------+---------------------+------+-----+---------+-------+
7 rows in set (0.00 sec)
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表操作-删除

删除表
DROP TABLE [ IF EXISTS ] 表名;
删除指定表, 并重新创建表
TRUNCATE TABLE 表名;
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实际操作

DROP TABLE IF EXISTS employee;
TRUNCATE TABLE employee;
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DML(数据操作语言)

DML英文全称是Data Manipulation Language(数据操作语言),用来对数据库中表的数据记录进行增、删、改操作

  • 添加数据(INSERT)
  • 修改数据(UPDATE)
  • 删除数据(DELETE)

添加数据

指定字段:
INSERT INTO 表名 (字段名1, 字段名2, ...) VALUES (值1, 值2, ...);
全部字段:
INSERT INTO 表名 VALUES (值1, 值2, ...);

批量添加数据:
INSERT INTO 表名 (字段名1, 字段名2, ...) VALUES (值1, 值2, ...), (值1, 值2, ...), (值1, 值2, ...);
INSERT INTO 表名 VALUES (值1, 值2, ...), (值1, 值2, ...), (值1, 值2, ...);
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实际操作

create table emp(
    id int comment '编号',
    workno varchar(10) comment '工号',
    name varchar(10) comment '姓名',
    gender char(1) comment '性别',
    age tinyint unsigned comment '年龄',
    idcard char(18) comment '身份证号',
    workaddress varchar(50) comment '工作地址',
    entrydate date comment '入职时间'
)comment '员工表';

insert into employee(id,workno,name,gender,age,idcard,entrydate) values(1,'1','Job','m',10,'123456789012345678','2000-01-01');
insert into employee(id,workno,name,gender,age,idcard,entrydate) values(1,'1','Job','m',-1,'123456789012345678','2000-01-01');
insert into employee values(2,'2','Jason','m',18,'123456789012345670','2005-01-01');
insert into employee values(3,'3','Steve','m',38,'123456789012345670','2005-01-01'),(4,'4','Helen','w',18,'123456789012345670','2005-01-01');

mysql> insert into employee(id,workno,name,gender,age,idcard,entrydate) values(1,'1','Job','m',10,'123456789012345678','2000-01-01');
Query OK, 1 row affected (0.01 sec)

mysql> select * from employee;
+------+--------+------+--------+------+--------------------+------------+
| id   | workno | name | gender | age  | idcard             | entrydate  |
+------+--------+------+--------+------+--------------------+------------+
|    1 | 1      | Job  | m      |   10 | 123456789012345678 | 2000-01-01 |
+------+--------+------+--------+------+--------------------+------------+
1 row in set (0.00 sec)

mysql> insert into employee(id,workno,name,gender,age,idcard,entrydate) values(1,'1','Job','m',-1,'123456789012345678','2000-01-01');
ERROR 1264 (22003): Out of range value for column 'age' at row 1
mysql> insert into employee values(2,'2','Jason','m',18,'123456789012345670','2005-01-01');
Query OK, 1 row affected (0.01 sec)

mysql> insert into employee values(3,'3','Steve','m',38,'123456789012345670','2005-01-01'),(4,'4','Helen','w',18,'123456789012345670','2005-01-01');
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from employee;
+------+--------+-------+--------+------+--------------------+------------+
| id   | workno | name  | gender | age  | idcard             | entrydate  |
+------+--------+-------+--------+------+--------------------+------------+
|    1 | 1      | Job   | m      |   10 | 123456789012345678 | 2000-01-01 |
|    2 | 2      | Jason | m      |   18 | 123456789012345670 | 2005-01-01 |
|    3 | 3      | Steve | m      |   38 | 123456789012345670 | 2005-01-01 |
|    4 | 4      | Helen | w      |   18 | 123456789012345670 | 2005-01-01 |
+------+--------+-------+--------+------+--------------------+------------+
4 rows in set (0.00 sec)
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注意事项

  • 插入数据时,指定的字段顺序需要与值的顺序是一一对应的。
  • 字符串和日期类型数据应该包含在引号中
  • 插入的数据大小应该在字段的规定范围内

更新数据

修改数据:
UPDATE 表名 SET 字段名1 = 值1, 字段名2 = 值2, ... [ WHERE 条件 ];
例:
UPDATE emp SET name = 'Jack' WHERE id = 1;
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实际操作

-- A. 修改id为1的数据,将name修改为Jack
    update employee set name = 'Jack' where id = 1;
-- B. 修改id为1的数据, 将name修改为Lucky, gender修改为 w
    update employee set name = 'Lucky' , gender = 'w' where id = 1;
-- C. 将所有的员工入职日期修改为 2008-01-01
    update employee set entrydate = '2008-01-01';
	
mysql> update employee set name = 'Jack' where id = 1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from employee;
+------+--------+-------+--------+------+--------------------+------------+
| id   | workno | name  | gender | age  | idcard             | entrydate  |
+------+--------+-------+--------+------+--------------------+------------+
|    1 | 1      | Jack  | m      |   10 | 123456789012345678 | 2000-01-01 |
|    2 | 2      | Jason | m      |   18 | 123456789012345670 | 2005-01-01 |
|    3 | 3      | Steve | m      |   38 | 123456789012345670 | 2005-01-01 |
|    4 | 4      | Helen | w      |   18 | 123456789012345670 | 2005-01-01 |
+------+--------+-------+--------+------+--------------------+------------+
4 rows in set (0.00 sec)

mysql> update employee set name = 'Lucky' , gender = 'w' where id = 1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from employee;
+------+--------+-------+--------+------+--------------------+------------+
| id   | workno | name  | gender | age  | idcard             | entrydate  |
+------+--------+-------+--------+------+--------------------+------------+
|    1 | 1      | Lucky | w      |   10 | 123456789012345678 | 2000-01-01 |
|    2 | 2      | Jason | m      |   18 | 123456789012345670 | 2005-01-01 |
|    3 | 3      | Steve | m      |   38 | 123456789012345670 | 2005-01-01 |
|    4 | 4      | Helen | w      |   18 | 123456789012345670 | 2005-01-01 |
+------+--------+-------+--------+------+--------------------+------------+
4 rows in set (0.00 sec)

mysql> update employee set entrydate = '2008-01-01';
Query OK, 4 rows affected (0.00 sec)
Rows matched: 4  Changed: 4  Warnings: 0

mysql> select * from employee;
+------+--------+-------+--------+------+--------------------+------------+
| id   | workno | name  | gender | age  | idcard             | entrydate  |
+------+--------+-------+--------+------+--------------------+------------+
|    1 | 1      | Lucky | w      |   10 | 123456789012345678 | 2008-01-01 |
|    2 | 2      | Jason | m      |   18 | 123456789012345670 | 2008-01-01 |
|    3 | 3      | Steve | m      |   38 | 123456789012345670 | 2008-01-01 |
|    4 | 4      | Helen | w      |   18 | 123456789012345670 | 2008-01-01 |
+------+--------+-------+--------+------+--------------------+------------+
4 rows in set (0.00 sec)

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注意事项

  • 修改语句的条件可以有,也可以没有,如果没有条件,则会修改整张表的所有数据。

删除数据

删除数据:
DELETE FROM 表名 [ WHERE 条件 ];
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实际操作

-- A. 删除gender为女的员工
      delete from employee where gender = 'w';
-- B. 删除所有员工
      delete from employee;
	
mysql> select * from employee;
+------+--------+-------+--------+------+--------------------+------------+
| id   | workno | name  | gender | age  | idcard             | entrydate  |
+------+--------+-------+--------+------+--------------------+------------+
|    1 | 1      | Lucky | w      |   10 | 123456789012345678 | 2008-01-01 |
|    2 | 2      | Jason | m      |   18 | 123456789012345670 | 2008-01-01 |
|    3 | 3      | Steve | m      |   38 | 123456789012345670 | 2008-01-01 |
|    4 | 4      | Helen | w      |   18 | 123456789012345670 | 2008-01-01 |
+------+--------+-------+--------+------+--------------------+------------+
4 rows in set (0.00 sec)

mysql> delete from employee where gender = 'w';
Query OK, 2 rows affected (0.01 sec)

mysql> select * from employee;
+------+--------+-------+--------+------+--------------------+------------+
| id   | workno | name  | gender | age  | idcard             | entrydate  |
+------+--------+-------+--------+------+--------------------+------------+
|    2 | 2      | Jason | m      |   18 | 123456789012345670 | 2008-01-01 |
|    3 | 3      | Steve | m      |   38 | 123456789012345670 | 2008-01-01 |
+------+--------+-------+--------+------+--------------------+------------+
2 rows in set (0.00 sec)

mysql> delete from employee;
Query OK, 2 rows affected (0.01 sec)

mysql> select * from employee;
Empty set (0.00 sec)
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注意事项

  • DELETE 语句的条件可以有,也可以没有,如果没有条件,则会删除整张表的所有数据。
  • DELETE 语句不能删除某一个字段的值(可以使用UPDATE,将该字段值置为NULL即可)。

DQL(数据查询语言)

DQL英文全称是Data Query Language(数据查询语言),数据查询语言,用来查询数据库中表的记录。

查询关键字: SELECT

在一个正常的业务系统中,查询操作的频次是要远高于增删改的,当我们去访问企业官网、电商网站,在这些网站中我们所看到的数据,实际都是需要从数据库中查询并展示的。而且在查询的过程中,可能还会涉及到条件、排序、分页等操作。

数据源 emp.sql 为基础数据。

基本语法

SELECT
	字段列表
FROM
	表名字段
WHERE
	条件列表
GROUP BY
	分组字段列表
HAVING
	分组后的条件列表
ORDER BY
	排序字段列表
LIMIT
	分页参数
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可以将上述完整语法进行拆分,分为以下几个部分:

  • 基本查询(不带任何条件)
  • 条件查询(WHERE)
  • 聚合函数(count、max、min、avg、sum)
  • 分组查询(group by)
  • 排序查询(order by)
  • 分页查询(limit)

基础查询

查询多个字段:
SELECT 字段1, 字段2, 字段3, ... FROM 表名;
SELECT * FROM 表名;

设置别名:
SELECT 字段1 [ AS 别名1 ], 字段2 [ AS 别名2 ], 字段3 [ AS 别名3 ], ... FROM 表名;
SELECT 字段1 [ 别名1 ], 字段2 [ 别名2 ], 字段3 [ 别名3 ], ... FROM 表名;

去除重复记录:
SELECT DISTINCT 字段列表 FROM 表名;

转义:
SELECT * FROM 表名 WHERE name LIKE '/_张三' ESCAPE '/'
/ 之后的\_不作为通配符
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实际操作

-- A. 查询指定字段 name, workno, age并返回
       select name,workno,age from emp;
-- B. 查询返回所有字段
       select id ,workno,name,gender,age,idcard,workaddress,entrydate from emp;
       select * from emp;
-- C. 查询所有员工的工作地址,起别名
       select workaddress as '工作地址' from emp;
--     -- as可以省略
       select workaddress '工作地址' from emp;
-- D. 查询公司员工的上班地址有哪些(不要重复)
       select distinct workaddress '工作地址' from emp;

mysql> select name,workno,age from emp;
+-----------+--------+------+
| name      | workno | age  |
+-----------+--------+------+
| 柳岩    | 00001  |   20 |
| 张无忌 | 00002  |   18 |
| 韦一笑 | 00003  |   38 |
| 赵敏    | 00004  |   18 |
| 小昭    | 00005  |   16 |
| 杨逍    | 00006  |   28 |
| 范瑶    | 00007  |   40 |
| 黛绮丝 | 00008  |   38 |
| 范凉凉 | 00009  |   45 |
| 陈友谅 | 00010  |   53 |
| 张士诚 | 00011  |   55 |
| 常遇春 | 00012  |   32 |
| 张三丰 | 00013  |   88 |
| 灭绝    | 00014  |   65 |
| 胡青牛 | 00015  |   70 |
| 周芷若 | 00016  |   18 |
+-----------+--------+------+

mysql> select * from emp;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    3 | 00003  | 韦一笑 ||   38 | 123456789712345670 | 上海      | 2005-08-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|    8 | 00008  | 黛绮丝 ||   38 | 123456157123645670 | 天津      | 2015-05-01 |
|    9 | 00009  | 范凉凉 ||   45 | 123156789012345678 | 北京      | 2010-04-01 |
|   10 | 00010  | 陈友谅 ||   53 | 123456789012345670 | 上海      | 2011-01-01 |
|   11 | 00011  | 张士诚 ||   55 | 123567897123465670 | 江苏      | 2015-05-01 |
|   12 | 00012  | 常遇春 ||   32 | 123446757152345670 | 北京      | 2004-02-01 |
|   13 | 00013  | 张三丰 ||   88 | 123656789012345678 | 江苏      | 2020-11-01 |
|   14 | 00014  | 灭绝    ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
16 rows in set (0.00 sec)

mysql> select workaddress as '工作地址' from emp;
+--------+
| 工作地址|
+--------+
| 北京 |
| 北京 |
| 上海 |
| 北京 |
| 上海 |
| 北京 |
| 北京 |
| 天津 |
| 北京 |
| 上海 |
| 江苏 |
| 北京 |
| 江苏 |
| 西安 |
| 西安 |
| 北京 |
+--------+
16 rows in set (0.00 sec)

mysql> select distinct workaddress '工作地址' from emp;
+--------+
| 工作地址|
+--------+
| 北京 |
| 上海 |
| 天津 |
| 江苏 |
| 西安 |
+--------+
5 rows in set (0.00 sec)

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条件查询

语法: SELECT 字段列表 FROM 表名 WHERE 条件列表;

条件:

比较运算符 功能
> 大于
>= 大于等于
< 小于
<= 小于等于
= 等于
<> 或 != 不等于
BETWEEN ... AND ... 在某个范围内(含最小、最大值)
IN(...) 在in之后的列表中的值,多选一
LIKE 占位符 模糊匹配(_匹配单个字符,%匹配任意个字符)
IS NULL 是NULL
逻辑运算符 功能
AND 或 && 并且(多个条件同时成立)
OR 或 || 或者(多个条件任意一个成立)
NOT 或 ! 非,不是

实际操作

-- A. 查询年龄等于 88 的员工
  	select * from emp where age = 88;
-- B. 查询年龄小于 20 的员工信息
 	select * from emp where age < 20;
-- C. 查询年龄小于等于 20 的员工信息
  	select * from emp where age <= 20;
-- D. 查询没有身份证号的员工信息
 	select * from emp where idcard is null;
-- E. 查询有身份证号的员工信息
 	select * from emp where idcard is not null;
-- F. 查询年龄不等于 88 的员工信息
  	select * from emp where age != 88;
  	select * from emp where age <> 88;
-- G. 查询年龄在15岁(包含) 到 20岁(包含)之间的员工信息
 	select * from emp where age >= 15 && age <= 20;
  	select * from emp where age >= 15 and age <= 20;
-- H. 查询性别为 女 且年龄小于 25岁的员工信息
 	select * from emp where gender = '女' and age < 25;
-- I. 查询年龄等于18 或 20 或 40 的员工信息
 	select * from emp where age = 18 or age = 20 or age =40;
 	select * from emp where age in(18,20,40);
-- J. 查询姓名为两个字的员工信息 _ %
 	select * from emp where name like '__';
-- K. 查询身份证号最后一位是X的员工信息
 	select * from emp where idcard like '%X';
 	select * from emp where idcard like '_________________X';

mysql> select * from emp where age = 88;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|   13 | 00013  | 张三丰 ||   88 | 123656789012345678 | 江苏      | 2020-11-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
1 row in set (0.00 sec)

mysql> select * from emp where age < 20;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
4 rows in set (0.00 sec)

mysql> select * from emp where age <= 20;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
5 rows in set (0.00 sec)

mysql> select * from emp where idcard is null;
+------+--------+-----------+--------+------+--------+-------------+------------+
| id   | workno | name      | gender | age  | idcard | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------+-------------+------------+
|   16 | 00016  | 周芷若 ||   18 | NULL   | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------+-------------+------------+
1 row in set (0.00 sec)

mysql> select * from emp where idcard is not null;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    3 | 00003  | 韦一笑 ||   38 | 123456789712345670 | 上海      | 2005-08-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|    8 | 00008  | 黛绮丝 ||   38 | 123456157123645670 | 天津      | 2015-05-01 |
|    9 | 00009  | 范凉凉 ||   45 | 123156789012345678 | 北京      | 2010-04-01 |
|   10 | 00010  | 陈友谅 ||   53 | 123456789012345670 | 上海      | 2011-01-01 |
|   11 | 00011  | 张士诚 ||   55 | 123567897123465670 | 江苏      | 2015-05-01 |
|   12 | 00012  | 常遇春 ||   32 | 123446757152345670 | 北京      | 2004-02-01 |
|   13 | 00013  | 张三丰 ||   88 | 123656789012345678 | 江苏      | 2020-11-01 |
|   14 | 00014  | 灭绝    ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
15 rows in set (0.00 sec)

mysql> select * from emp where age != 88;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    3 | 00003  | 韦一笑 ||   38 | 123456789712345670 | 上海      | 2005-08-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|    8 | 00008  | 黛绮丝 ||   38 | 123456157123645670 | 天津      | 2015-05-01 |
|    9 | 00009  | 范凉凉 ||   45 | 123156789012345678 | 北京      | 2010-04-01 |
|   10 | 00010  | 陈友谅 ||   53 | 123456789012345670 | 上海      | 2011-01-01 |
|   11 | 00011  | 张士诚 ||   55 | 123567897123465670 | 江苏      | 2015-05-01 |
|   12 | 00012  | 常遇春 ||   32 | 123446757152345670 | 北京      | 2004-02-01 |
|   14 | 00014  | 灭绝    ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
15 rows in set (0.00 sec)

mysql> select * from emp where age >= 15 && age <= 20;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
5 rows in set (0.00 sec)

mysql> select * from emp where gender = '女' and age < 25;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
5 rows in set (0.00 sec)

mysql> select * from emp where age in(18,20,40);
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
5 rows in set (0.00 sec)

mysql> select * from emp where name like '__';
+------+--------+--------+--------+------+--------------------+-------------+------------+
| id   | workno | name   | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+--------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩 ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    4 | 00004  | 赵敏 ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭 ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|    6 | 00006  | 杨逍 ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|    7 | 00007  | 范瑶 ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|   14 | 00014  | 灭绝 ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
+------+--------+--------+--------+------+--------------------+-------------+------------+
6 rows in set (0.00 sec)

mysql> select * from emp where idcard like '%X';
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
2 rows in set (0.00 sec)
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聚合查询(聚合函数)

将一列数据作为一个整体,进行纵向计算 。

常见聚合函数:

函数 功能
count 统计数量
max 最大值
min 最小值
avg 平均值
sum 求和

语法:

SELECT 聚合函数(字段列表) FROM 表名;

注意 : NULL值是不参与所有聚合函数运算的。

案例:

-- A. 统计该企业员工数量
   	select count(*) from emp; -- 统计的是总记录数
  	select count(idcard) from emp; -- 统计的是idcard字段不为null的记录数

-- 对于count聚合函数,统计符合条件的总记录数,还可以通过 count(数字/字符串)的形式进行统计
-- 查询,比如:
 	select count(1) from emp;
	
-- 对于count(*) 、count(字段)、 count(1) 的具体原理,我们在进阶篇中SQL优化部分会详细讲解,此处大家只需要知道如何使用即可。

-- B. 统计该企业员工的平均年龄
 	select avg(age) from emp;
-- C. 统计该企业员工的最大年龄
 	select max(age) from emp;
-- D. 统计该企业员工的最小年龄
  	select min(age) from emp;
-- E. 统计西安地区员工的年龄之和
  	select sum(age) from emp where workaddress = '西安';

mysql> select count(*) from emp;
+----------+
| count(*) |
+----------+
|       16 |
+----------+
1 row in set (0.00 sec)

mysql> select count(1) from emp;
+----------+
| count(1) |
+----------+
|       16 |
+----------+
1 row in set (0.00 sec)

mysql> select avg(age) from emp;
+----------+
| avg(age) |
+----------+
|  40.1250 |
+----------+
1 row in set (0.00 sec)

mysql> select max(age) from emp;
+----------+
| max(age) |
+----------+
|       88 |
+----------+
1 row in set (0.00 sec)

mysql> select min(age) from emp;
+----------+
| min(age) |
+----------+
|       16 |
+----------+
1 row in set (0.00 sec)

mysql> select sum(age) from emp where workaddress = '西安';
+----------+
| sum(age) |
+----------+
|       135|
+----------+
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分组查询

语法:

SELECT 字段列表 FROM 表名 [ WHERE 条件 ] GROUP BY 分组字段名 [ HAVING 分组后的过滤条件 ];
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where 和 having 的区别:

  • 执行时机不同:where是分组之前进行过滤,不满足where条件不参与分组;having是分组后对结果进行过滤。
  • 判断条件不同:where不能对聚合函数进行判断,而having可以。

例子:

-- A. 根据性别分组,统计男性和女性数量
   	select gender, count(*) from emp group by gender;
-- B. 根据性别分组,统计男性和女性的平均年龄
   	select gender, avg(age) from emp group by gender;
-- C. 年龄小于45,并根据工作地址分组,获取员工数量大于等于3的工作地址
   	select workaddress, count(*) address_count from emp where age < 45 group by workaddress having address_count >= 3;
-- D. 统计各个工作地址上班的男性及女性员工的数量
   	select workaddress, gender, count(*) '数量' from emp group by gender , workaddress


mysql> select gender, count(*) from emp group by gender;
+--------+----------+
| gender | count(*) |
+--------+----------+
||        7 |
||        9 |
+--------+----------+
2 rows in set (0.00 sec)

mysql> select gender, avg(age) from emp group by gender;
+--------+----------+
| gender | avg(age) |
+--------+----------+
||  31.4286 |
||  46.8889 |
+--------+----------+
2 rows in set (0.00 sec)

mysql> select workaddress, count(*) address_count from emp where age < 45 group by workaddress having address_count >= 3;
+-------------+---------------+
| workaddress | address_count |
+-------------+---------------+
| 北京      |             7 |
+-------------+---------------+
1 row in set (0.01 sec)

mysql> select workaddress, gender, count(*) '数量' from emp group by gender , workaddress
    -> ;
+-------------+--------+-------+
| workaddress | gender | 数量 |
+-------------+--------+-------+
| 上海      ||     1 |
| 北京      ||     4 |
| 天津      ||     1 |
| 西安      ||     1 |
| 上海      ||     2 |
| 北京      ||     4 |
| 江苏      ||     2 |
| 西安      ||     1 |
+-------------+--------+-------+
8 rows in set (0.00 sec)
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注意事项

  • 执行顺序:where > 聚合函数 > having
  • 分组之后,查询的字段一般为聚合函数和分组字段,查询其他字段无任何意义
  • 支持多字段分组, 具体语法为 : group by columnA,columnB

排序查询

排序在日常开发中是非常常见的一个操作,有升序排序,也有降序排序。例如,商品价格、销量排序等

语法:

SELECT 字段列表 FROM 表名 ORDER BY 字段1 排序方式1, 字段2 排序方式2;
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排序方式:

  • ASC: 升序(默认)
  • DESC: 降序

注意事项:

  • 如果是升序, 可以不指定排序方式ASC ;
  • 如果是多字段排序,当第一个字段值相同时,才会根据第二个字段进行排序 ;

例子:

-- A. 根据年龄对公司的员工进行升序排序
 	  select * from emp order by age asc;
 	  select * from emp order by age;
-- B. 根据入职时间, 对员工进行降序排序
 	  select * from emp order by entrydate desc;
-- C. 根据年龄对公司的员工进行升序排序 , 年龄相同 , 再按照入职时间进行降序排序
 	  select * from emp order by age asc , entrydate desc;

mysql> select * from emp order by age asc;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|   12 | 00012  | 常遇春 ||   32 | 123446757152345670 | 北京      | 2004-02-01 |
|    3 | 00003  | 韦一笑 ||   38 | 123456789712345670 | 上海      | 2005-08-01 |
|    8 | 00008  | 黛绮丝 ||   38 | 123456157123645670 | 天津      | 2015-05-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|    9 | 00009  | 范凉凉 ||   45 | 123156789012345678 | 北京      | 2010-04-01 |
|   10 | 00010  | 陈友谅 ||   53 | 123456789012345670 | 上海      | 2011-01-01 |
|   11 | 00011  | 张士诚 ||   55 | 123567897123465670 | 江苏      | 2015-05-01 |
|   14 | 00014  | 灭绝    ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
|   13 | 00013  | 张三丰 ||   88 | 123656789012345678 | 江苏      | 2020-11-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
16 rows in set (0.00 sec)

mysql> select * from emp order by entrydate desc;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|   13 | 00013  | 张三丰 ||   88 | 123656789012345678 | 江苏      | 2020-11-01 |
|   14 | 00014  | 灭绝    ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
|    8 | 00008  | 黛绮丝 ||   38 | 123456157123645670 | 天津      | 2015-05-01 |
|   11 | 00011  | 张士诚 ||   55 | 123567897123465670 | 江苏      | 2015-05-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
|   10 | 00010  | 陈友谅 ||   53 | 123456789012345670 | 上海      | 2011-01-01 |
|    9 | 00009  | 范凉凉 ||   45 | 123156789012345678 | 北京      | 2010-04-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    3 | 00003  | 韦一笑 ||   38 | 123456789712345670 | 上海      | 2005-08-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|   12 | 00012  | 常遇春 ||   32 | 123446757152345670 | 北京      | 2004-02-01 |
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
16 rows in set (0.00 sec)

mysql> select * from emp order by age asc , entrydate desc;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|   12 | 00012  | 常遇春 ||   32 | 123446757152345670 | 北京      | 2004-02-01 |
|    8 | 00008  | 黛绮丝 ||   38 | 123456157123645670 | 天津      | 2015-05-01 |
|    3 | 00003  | 韦一笑 ||   38 | 123456789712345670 | 上海      | 2005-08-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|    9 | 00009  | 范凉凉 ||   45 | 123156789012345678 | 北京      | 2010-04-01 |
|   10 | 00010  | 陈友谅 ||   53 | 123456789012345670 | 上海      | 2011-01-01 |
|   11 | 00011  | 张士诚 ||   55 | 123567897123465670 | 江苏      | 2015-05-01 |
|   14 | 00014  | 灭绝    ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
|   13 | 00013  | 张三丰 ||   88 | 123656789012345678 | 江苏      | 2020-11-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
16 rows in set (0.00 sec)

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注意事项

如果是多字段排序,当第一个字段值相同时,才会根据第二个字段进行排序

分页查询

分页操作在业务系统开发时,也是非常常见的一个功能,我们在网站中看到的各种各样的分页条,后台都需要借助于数据库的分页操作。

语法:

SELECT 字段列表 FROM 表名 LIMIT 起始索引, 查询记录数;
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注意事项:

  • 起始索引从0开始,起始索引 = (查询页码 - 1)* 每页显示记录数。
  • 分页查询是数据库的方言,不同的数据库有不同的实现,MySQL中是LIMIT。
  • 如果查询的是第一页数据,起始索引可以省略,直接简写为 limit 10。

例子:

-- A. 查询第1页员工数据, 每页展示10条记录
   	select * from emp limit 0,10;
   	select * from emp limit 10;
-- B. 查询第2页员工数据, 每页展示10条记录 --------> (页码-1)*页展示记录数
   	select * from emp limit 10,10

mysql> select * from emp limit 0,10;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|    1 | 00001  | 柳岩    ||   20 | 123456789012345678 | 北京      | 2000-01-01 |
|    2 | 00002  | 张无忌 ||   18 | 123456789012345670 | 北京      | 2005-09-01 |
|    3 | 00003  | 韦一笑 ||   38 | 123456789712345670 | 上海      | 2005-08-01 |
|    4 | 00004  | 赵敏    ||   18 | 123456757123845670 | 北京      | 2009-12-01 |
|    5 | 00005  | 小昭    ||   16 | 123456769012345678 | 上海      | 2007-07-01 |
|    6 | 00006  | 杨逍    ||   28 | 12345678931234567X | 北京      | 2006-01-01 |
|    7 | 00007  | 范瑶    ||   40 | 123456789212345670 | 北京      | 2005-05-01 |
|    8 | 00008  | 黛绮丝 ||   38 | 123456157123645670 | 天津      | 2015-05-01 |
|    9 | 00009  | 范凉凉 ||   45 | 123156789012345678 | 北京      | 2010-04-01 |
|   10 | 00010  | 陈友谅 ||   53 | 123456789012345670 | 上海      | 2011-01-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
10 rows in set (0.00 sec)

mysql> select * from emp limit 10,10;
+------+--------+-----------+--------+------+--------------------+-------------+------------+
| id   | workno | name      | gender | age  | idcard             | workaddress | entrydate  |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
|   11 | 00011  | 张士诚 ||   55 | 123567897123465670 | 江苏      | 2015-05-01 |
|   12 | 00012  | 常遇春 ||   32 | 123446757152345670 | 北京      | 2004-02-01 |
|   13 | 00013  | 张三丰 ||   88 | 123656789012345678 | 江苏      | 2020-11-01 |
|   14 | 00014  | 灭绝    ||   65 | 123456719012345670 | 西安      | 2019-05-01 |
|   15 | 00015  | 胡青牛 ||   70 | 12345674971234567X | 西安      | 2018-04-01 |
|   16 | 00016  | 周芷若 ||   18 | NULL               | 北京      | 2012-06-01 |
+------+--------+-----------+--------+------+--------------------+-------------+------------+
6 rows in set (0.00 sec)
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执行顺序

DQL语句在执行时的执行顺序,也就是先执行那一部分,后执行那一部分

FROM -> WHERE -> GROUP BY -> HAVING > SELECT -> ORDER BY -> LIMIT

查询年龄大于15的员工姓名、年龄,并根据年龄进行升序排序。
	select name , age from emp where age <= 35 order by age asc , entrydate desc;

在查询时,我们给emp表起一个别名 e,然后在selectwhere中使用该别名。
	select e.name , e.age from emp e where e.age > 15 order by age asc;

执行上述SQL语句后,我们看到依然可以正常的查询到结果,此时就说明: from 先执行, 然后
whereselect 执行。那 whereselect 到底哪个先执行呢?

此时我们可以给select后面的字段起别名,然后在 where 中使用这个别名,然后看看是否可以执行成功。执行上述SQL报错了:eage找不到
	select e.name ename , e.age eage from emp e where eage > 15 order by age asc;

由此我们可以得出结论: from 先执行,然后执行 where , 再执行selectselect e.name ename , e.age eage from emp e where e.age > 15 order by eage asc;

接下来,我们再执行如下SQL语句,查看执行效果:结果执行成功。 

那么也就验证了: order by 是在select 语句之后执行的。

综上所述,我们可以看到DQL语句的执行顺序为: from ... where ... group by ...having ... select ... order by ... limit ...
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DCL

DCL英文全称是Data Control Language(数据控制语言),用来管理数据库用户、控制数据库的访问权限。

管理用户

查询用户:

USE mysql;
SELECT * FROM user;
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  • Host代表当前用户访问的主机, 如果为localhost, 仅代表只能够在当前本机访问,是不可以 远程访问的。
  • User代表的是访问该数据库的用户名。在MySQL中需要通过Host和User来唯一标识一个用户。
创建用户:
CREATE USER '用户名'@'主机名' IDENTIFIED BY '密码';

修改用户密码:
ALTER USER '用户名'@'主机名' IDENTIFIED WITH mysql_native_password BY '新密码';

删除用户:
DROP USER '用户名'@'主机名';
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例子:

-- 创建用户 test , 只能够在当前主机localhost访问, 密码123456;
create user 'test'@'localhost' identified by '123456';

-- 创建用户 myuser , 可以在任意主机访问该数据库, 密码123456 ;
create user 'myuser'@'%' identified by '123456';

-- 修改用户 myuser 的访问密码为 1234 ;
alter user 'myuser'@'%' identified with mysql_native_password by '1234';

-- 删除test@localhost用户
drop user 'test'@'localhost';
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注意事项

  • 主机名可以使用 % 通配

权限控制

常用权限:

权限 说明
ALL, ALL PRIVILEGES 所有权限
SELECT 查询数据
INSERT 插入数据
UPDATE 修改数据
DELETE 删除数据
ALTER 修改表
DROP 删除数据库/表/视图
CREATE 创建数据库/表

更多权限请看权限一览表

查询权限:
SHOW GRANTS FOR '用户名'@'主机名';

授予权限:
GRANT 权限列表 ON 数据库名.表名 TO '用户名'@'主机名';

撤销权限:
REVOKE 权限列表 ON 数据库名.表名 FROM '用户名'@'主机名';
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例子:

-- 查询权限
show grants for 'docker'@'%';

-- 授予权限
grant all on docker_mysql.* to 'docker'@'%';

-- 撤销权限
revoke all on docker_mysql.* from 'docker'@'%';
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注意事项

  • 多个权限, 用逗号分隔
  • 授权时,数据库名和表名可以用 * 进行通配,代表所有