目录

1.as和distinct关键字

1.1 as关键字

1.2 distinct关键字

1.3 小结

2.where条件查询

3. 排序

4.分页查询

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1.as和distinct关键字

1.1 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。

1. 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

2. 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

   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。

1.2 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

1.3 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。

2.where条件查询

where语句支持的运算符:

1. 比较运算符，>,<,>=,<=,!=
2. 逻辑运算符, and,or,not
3. 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
4. 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
5. 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

3. 排序

排序查询语法：

select * from 表名 order by 列1 asc|desc [,列2 asc|desc,...]

语法说明:

1. 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
2. asc从小到大排列，即升序
3. desc从大到小排序，即降序
4. 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

4.分页查询

分页查询的介绍

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

说明:

1. limit是分页查询关键字
2. start表示开始行索引，默认是0
3. 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

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