MySQL - 数据库使用技巧（去重、排序、条件和分页查询）
as关键字
as关键字
在使用SQL语句显示结果的时候，往往在屏幕显示的字段名并不具备良好的可读性，此时可以使用 as 给字段起一个别名。
使用 as 给字段起别名
mysql> select id as 序号, name as 名字 from teacher;
+------+-------+
| 序号 | 名字 |
+------+-------+
| 1 | Helen |
| 2 | Jason |
+------+-------+
2 rows in set
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可以通过 as 给表起别名
-- 如果是单表查询 可以省略表名
select id, name from student;
-- 表名.字段名
select students.id,students.name from student;
-- 可以通过 as 给表起别名
select s.id,s.name from student as s;
mysql> select s.id,s.name from student as s;
+----+-------+
| id | name |
+----+-------+
| 1 | Jason |
| 4 | Steve |
+----+-------+
2 rows in set
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说明: 在这里给表起别名看起来并没有什么意义,然而并不是这样的。在后期学习自连接的时候，必须要对表起别名。
# distinct关键字
distinct可以去除重复数据行。
select distinct 列1,... from 表名;
例： 查询班级中学生的性别
select name, id from student;
-- 看到了很多重复数据 想要对其中重复数据行进行去重操作可以使用 distinct
select distinct name, id from student;
mysql> select name,sex from student;
+-------+-----+
| name | sex |
+-------+-----+
| Jason | 男 |
| Steve | 男 |
| Jason | 男 |
| Steve | 男 |
+-------+-----+
4 rows in set
mysql> select distinct name,sex from student;
+-------+-----+
| name | sex |
+-------+-----+
| Jason | 男 |
| Steve | 男 |
+-------+-----+
2 rows in set
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小结
- as 关键字可以给表中字段 或者 表名起别名
- distinct 关键字可以去除重复数据行。
where条件查询
where语句支持的运算符:
- 比较运算符，>,<,>=,<=,!=
- 逻辑运算符, and,or,not
- 模糊查询，like是模糊查询关键字，%表示任意多个任意字符，_表示一个任意字符
- 范围查询，between .. and .. 表示在一个连续的范围内查询，in 表示在一个非连续的范围内查询
- 空判断，判断为空使用: is null，判断非空使用: is not null
mysql> use mytestdata;
Database changed
mysql> select * from student;
+------+-------+-----+-------+
| id | name | age | sex |
+------+-------+-----+-------+
| 1001 | Jason | 25 | man |
| 1002 | Helen | 25 | woman |
| 1003 | Steve | 27 | man |
| 1004 | Hanna | 25 | woman |
| 1005 | Bob | 26 | man |
+------+-------+-----+-------+
5 rows in set
mysql> select * from student where sex='man';
+------+-------+-----+-----+
| id | name | age | sex |
+------+-------+-----+-----+
| 1001 | Jason | 25 | man |
| 1003 | Steve | 27 | man |
| 1005 | Bob | 26 | man |
+------+-------+-----+-----+
3 rows in set
mysql> select * from student where age between 26 and 27;
+------+-------+-----+-----+
| id | name | age | sex |
+------+-------+-----+-----+
| 1003 | Steve | 27 | man |
| 1005 | Bob | 26 | man |
+------+-------+-----+-----+
2 rows in set
mysql> select * from student where id >1004;
+------+------+-----+-----+
| id | name | age | sex |
+------+------+-----+-----+
| 1005 | Bob | 26 | man |
+------+------+-----+-----+
1 row in set
mysql> select * from student where name like 'H%';
+------+-------+-----+-------+
| id | name | age | sex |
+------+-------+-----+-------+
| 1002 | Helen | 25 | woman |
| 1004 | Hanna | 25 | woman |
+------+-------+-----+-------+
2 rows in set
mysql> select * from student where name like 'H_';
Empty set
mysql> select * from student where id >1003 and age>=
26;
+------+------+-----+-----+
| id | name | age | sex |
+------+------+-----+-----+
| 1005 | Bob | 26 | man |
+------+------+-----+-----+
1 row in set
mysql> select * from student where id is null;
Empty set
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排序
排序查询语法：
select * from 表名 order by 列1 asc|desc [,列2 asc|desc,...]
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语法说明:
- 先按照列1进行排序，如果列1的值相同时，则按照 列2 排序，以此类推
- asc从小到大排列，即升序
- desc从大到小排序，即降序
- 默认按照列值从小到大排列（即asc关键字）
以id降序:
mysql> select * from student order by id desc;
+------+-------+-----+-------+
| id | name | age | sex |
+------+-------+-----+-------+
| 1005 | Bob | 26 | man |
| 1004 | Hanna | 25 | woman |
| 1003 | Steve | 27 | man |
| 1002 | Helen | 25 | woman |
| 1001 | Jason | 25 | man |
+------+-------+-----+-------+
5 rows in set
以age,id降序:
mysql> select * from student order by age desc,id desc;
+------+-------+-----+-------+
| id | name | age | sex |
+------+-------+-----+-------+
| 1003 | Steve | 27 | man |
| 1005 | Bob | 26 | man |
| 1004 | Hanna | 25 | woman |
| 1002 | Helen | 25 | woman |
| 1001 | Jason | 25 | man |
+------+-------+-----+-------+
5 rows in set
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分页查询
当我们在天猫购物，浏览商品列表时，由于商品数据过多，一个页面无法完全展示，需要一页一页的进行显示，这就是分页查询。
分页查询的语法
select * from 表名 limit start,count
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说明:
- limit是分页查询关键字
- start表示开始行索引，默认是0
- count表示查询条数
查询前3条男生信息：
mysql> select * from student where sex='man' limit 0,3;
（或简写select * from student where sex='man' limit 3;）
+------+-------+-----+-----+
| id | name | age | sex |
+------+-------+-----+-----+
| 1001 | Jason | 25 | man |
| 1003 | Steve | 27 | man |
| 1005 | Bob | 26 | man |
+------+-------+-----+-----+
3 rows in set
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分页查询案例
已知每页显示m条数据，求第n页显示的数据
提示: 关键是求每页的开始行索引
查询学生表，获取第n页数据的SQL语句:
select * from student limit (n-1)*m,m
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