一、简介
DQL（Data Query Language）数据查询语言
用来查询数据库中表的数据。关键词：select、where等
二、简单查询
1）查询所有数据
select * from 表名;
2）查询指定字段
select 字段1,字段2,... from 表名;
3）别名查询
select 字段1 [as] 别名,字段2,... from 表名;
mysql> select id,name as 名字 from user;
+----+--------+
| id | 名字 |
+----+--------+
| 1 | 张三 |
| 2 | 大O |
| 3 | 小四 |
| 4 | 小小 |
+----+--------+
4）查询时去除重复值
select distinct 字段1,字段2,... from 表名;
重复的值只会出现一次。
mysql> select * from user1;
+------+--------+
| id | name |
+------+--------+
| 2 | 小O |
| 1 | 张三 |
| 2 | 大O |
| 3 | 小四 |
| 4 | 小小 |
| 2 | 大O |
+------+--------+
6 rows in set (0.00 sec)
mysql> select distinct id from user1;
+------+
| id |
+------+
| 2 |
| 1 |
| 3 |
| 4 |
+------+
4 rows in set (0.00 sec)
mysql> select distinct id,name from user1;
+------+--------+
| id | name |
+------+--------+
| 2 | 小O |
| 1 | 张三 |
| 2 | 大O |
| 3 | 小四 |
| 4 | 小小 |
+------+--------+
5 rows in set (0.00 sec)
5）查询结果参与计算
如：select 字段1+字段2 from 表名;、select 字段1*個定数值 from 表名;等。
mysql> select * from user;
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 1 | 张三 | 80 | 70 |
| 2 | 大O | 140 | 50 |
| 3 | 小四 | 150 | 30 |
| 4 | 小小 | 40 | 120 |
+----+--------+------+---------+
4 rows in set (0.00 sec)
mysql> select *,math+english 总分 from user;
+----+--------+------+---------+--------+
| id | name | math | english | 总分 |
+----+--------+------+---------+--------+
| 1 | 张三 | 80 | 70 | 150 |
| 2 | 大O | 140 | 50 | 190 |
| 3 | 小四 | 150 | 30 | 180 |
| 4 | 小小 | 40 | 120 | 160 |
+----+--------+------+---------+--------+
4 rows in set (0.01 sec)
三、简单条件查询
1）条件查询
语法：select [字段]... from 表名 where 条件;
| 运算符 | 说明 |
|---|---|
| > | 大于 |
| < | 小于 |
| <= | 小于或等于 |
| >= | 大于或等于 |
| = | 等于 |
| <>、!= | 不等于 |
| and | 多个条件同时满足 |
| or | 多个条件中满足其中一个 |
| between | 满足指定范围 |
| in | 在指定值里面 |
| not in | 不在指定值里面 |
mysql> select * from user where math > 100;
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 2 | 大O | 140 | 50 |
| 3 | 小四 | 150 | 30 |
+----+--------+------+---------+
2 rows in set (0.00 sec)
mysql> select * from user where english between 50 and 100;
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 1 | 张三 | 80 | 70 |
| 2 | 大O | 140 | 50 |
+----+--------+------+---------+
2 rows in set (0.00 sec)
2）模糊查询
语法：select [字段]... from 表名 like '通配符字符串';
MySQL通配符：
%：表示0个或者多个任意字符
_：表示一个任意字符
mysql> select * from user where name like "%小%";
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 3 | 小四 | 150 | 30 |
| 4 | 小小 | 40 | 120 |
+----+--------+------+---------+
2 rows in set (0.00 sec)
四、排序与分页
1）简单排序
语法：select [字段]... from 表名 order by 字段 [asc|desc],...
默认排序方式为：asc（升序）
mysql> select * from user order by math;
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 4 | 小小 | 40 | 120 |
| 1 | 张三 | 80 | 70 |
| 2 | 大O | 140 | 50 |
| 3 | 小四 | 150 | 30 |
+----+--------+------+---------+
4 rows in set (0.00 sec)
mysql> select * from user order by math desc;
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 3 | 小四 | 150 | 30 |
| 2 | 大O | 140 | 50 |
| 1 | 张三 | 80 | 70 |
| 4 | 小小 | 40 | 120 |
+----+--------+------+---------+
4 rows in set (0.00 sec)
2）分页
语法：select [字段]... from 表名 limit offset,length;或select [字段]... from 表名 limit length;
offset 是指偏移量，不指定的话默认为0。
length 需要显示的总记录数
mysql> select * from user;
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 1 | 张三 | 80 | 70 |
| 2 | 大O | 140 | 50 |
| 3 | 小四 | 150 | 30 |
| 4 | 小小 | 40 | 120 |
+----+--------+------+---------+
4 rows in set (0.00 sec)
mysql> select * from user limit 1,2;
+----+--------+------+---------+
| id | name | math | english |
+----+--------+------+---------+
| 2 | 大O | 140 | 50 |
| 3 | 小四 | 150 | 30 |
+----+--------+------+---------+
2 rows in set (0.00 sec)
五、参考
8.0官方文档：https://dev.mysql.com/doc/refman/8.0/en/