Mysql-DQL

mysql之DQL查询AS CONCAT LIKE的使用

select 列名1，列名2，... from 表名 [where 条件]

过滤掉重复的列值

select distinct 列名1 from 表名

-- 重复的列值只列出一次(去掉列值重复)
mysql> select distinct(password) from user;

链接concat

select concat(列名1，列名2) from 表名 concat_ws带分隔符

列起别名 as
select 列名1 as 别名，列名2 from 表名

模糊查询

select 列名 ... from 表名 where 列名 like '%字符串%';

mysql> select user_name from user where user_name like '%ng%';
+-----------+
| user_name |
+-----------+
| liming |
| zhangsan |
+-----------+

Mysql之DQL排序以及聚合函数

order by 字段 asc;
order by 字段 desc;

mysql> select user_name,id from user order by id asc;
+-----------+----+
| user_name | id |
+-----------+----+
| liming | 1 |
| zhangsan | 2 |
| 李华 | 3 |
+-----------+----+
3 rows in set (0.01 sec)

mysql> select user_name,id from user order by id desc;
+-----------+----+
| user_name | id |
+-----------+----+
| 李华 | 3 |
| zhangsan | 2 |
| liming | 1 |
+-----------+----+
3 rows in set (0.00 sec)

表中有多少条记录，某列的总和，平均值，最大值，最小值

mysql> select count(*) from user;
mysql> select sum(age) from user;
mysql> select avg(age) from user;
mysql> select max(age) from user;
mysql> select min(age) from user;

分组查询

user表的数据
+----+-----------+-----+----------+----------------+-------------+-------+------+
| id | user_name | sex | password | email | mobile | fee | age |
+----+-----------+-----+----------+----------------+-------------+-------+------+
| 1 | liming | 0 | 123435 | liming@163.com | 13666666666 | 12.21 | 28 |
| 2 | zhangsan | 1 | 229999 | zs@163.com | 13554442907 | 86.21 | 22 |
| 3 | 李华 | 0 | 123435 | lihua@163.com | 1366666666 | 99.12 | 18 |
| 4 | lisi | 1 | 2×××2 | lisi@163.com | 17376756841 | 32.18 | 35 |
| 5 | wangwu | 1 | 888888 | wangwu@163.com | 18511111122 | 38.69 | 26 |
+----+-----------+-----+----------+----------------+-------------+-------+------+

统计sex=1时的人数，sex=0时的人数

mysql> select sex,count(*) from user group by sex;
+-----+----------+
| sex | count(*) |
+-----+----------+
|   0 |        2 |
|   1 |        3 |
+-----+----------+

按照sex进行分组，哪组人数大于2

mysql> select sex from user group by sex having count(*)>2;
+-----+
| sex |
+-----+
|   1 |
+-----+

链接查询

内链接查询

-- where 可换成 on
mysql> select s.id,s.name,m.mark from student as s inner join mark as m where m.stu_id=s.id; 
+----+---------+------+
| id | name    | mark |
+----+---------+------+
|  2 | xiaoliu |   66 |
|  4 | xiaoli  |   77 |
|  5 | xiaopan |   88 |
+----+---------+------+
3 rows in set (0.00 sec)

mysql> select m.id,s.name,m.mark from student as s, mark as m where m.stu_id=s.id;
+----+---------+------+
| id | name    | mark |
+----+---------+------+
|  1 | xiaoliu |   66 |
|  2 | xiaoli  |   77 |
|  3 | xiaopan |   88 |
+----+---------+------+
3 rows in set (0.00 sec)

外链接查询(左链接查询，右链接查询)

左链接查询

mysql> select s.name,m.mark from student as s left join mark as m on m.stu_id=s.id;
+-----------+------+
| name      | mark |
+-----------+------+
| xiaoliu   |   66 |
| xiaoli    |   77 |
| xiaopan   |   88 |
| zhaozhang | NULL |
| xiaowang  | NULL |
+-----------+------+

右链接查询

mysql> select s.name,m.mark from student as s right join mark as m on m.stu_id=s.id;
+---------+------+
| name    | mark |
+---------+------+
| xiaoliu |   66 |
| xiaoli  |   77 |
| xiaopan |   88 |
+---------+------+

联合查询

三种方式实现：查询id=2或id=5的记录

mysql> select * from student where id in(2,5);
mysql> select * from student where id=2 or id=5;
mysql> select * from student where id=2 union all select * from student where id=5;

子查询

mysql> select id from student where id in(select stu_id from mark);
+----+
| id |
+----+
|  2 |
|  4 |
|  5 |
+----+

limit查询

mysql> select * from student limit 2;
+----+-----------+-----+
| id | name      | age |
+----+-----------+-----+
|  1 | zhaozhang |  26 |
|  2 | xiaoliu   |  27 |
+----+-----------+-----+

mysql> select * from student limit 3,2;
+----+---------+-----+
| id | name    | age |
+----+---------+-----+
|  4 | xiaoli  |  29 |
|  5 | xiaopan |  30 |
+----+---------+-----+