MySQL Quick Query-Subquery

MySQL quick check

Because I often forget some mysql statements, keywords, operations, etc. in my daily work and study, I recently took some time to write the following content about mysql. It's like a dictionary

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write in front

A subquery is a query nested in a select, insert, update, delete statement (or other subquery).

• Common subquery operations:
  • [not] in
  • any
  • all
  • [not] exists
  • Logical Operators
• According to the query results, it can be divided into:
  • Table subquery: Returns a table
  • Row self query: returns a row with one or more values
  • Column subquery: returns only one column for each row
  • Scalar self-query: returns a value

Note:

• Subqueries can be nested up to 32 levels
• order by can only be used outside the subquery
• Columns with data types such as text, blob, longtext, etc. cannot be retrieved.

Example

Table used in the example

mysql> desc user;
+-------+---------------+------+-----+---------+----------------+
| Field | Type          | Null | Key | Default | Extra          |
+-------+---------------+------+-----+---------+----------------+
| id    | int           | NO   | PRI | NULL    | auto_increment |
| name  | char(10)      | YES  |     | NULL    |                |
| sex   | enum('f','m') | YES  |     | NULL    |                |
+-------+---------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

mysql> desc commodity;
+---------+-----------+------+-----+--------------+----------------+
| Field   | Type      | Null | Key | Default      | Extra          |
+---------+-----------+------+-----+--------------+----------------+
| id      | int       | NO   | PRI | NULL         | auto_increment |
| price   | int       | NO   |     | NULL         |                |
| name    | char(128) | YES  |     | 匿名商品     |                |
| user_id | int       | YES  |     | NULL         |                |
+---------+-----------+------+-----+--------------+----------------+
4 rows in set (0.01 sec)

Use data that exists in the table

mysql> select * from user;
+----+--------+------+
| id | name   | sex  |
+----+--------+------+
|  1 | 铁子   | m    |
|  2 | 嘎子   | m    |
|  3 | 潘子   | m    |
|  4 | 翠花   | f    |
|  5 | 阿秀   | f    |
+----+--------+------+
5 rows in set (0.00 sec)

mysql> select * from commodity;
+----+-------+--------+---------+
| id | price | name   | user_id |
+----+-------+--------+---------+
|  1 |   100 | 手机   |       1 |
|  2 |   299 | 电脑   |       1 |
|  3 | 18990 | 假酒   |       3 |
|  4 | 18990 | 中华   |       3 |
|  5 |    18 | 中华   |       2 |
+----+-------+--------+---------+
5 rows in set (0.00 sec)

Use as an expression

# 查看id为3的user的购买数量
select name, 
	(select count(1) from commodity where user_id = 3) 购买数量
	from user
	where id = 3;
+--------+--------------+
| name   | 购买数量     |
+--------+--------------+
| 潘子   |            2 |
+--------+--------------+
1 row in set (0.00 sec)

Used with where clauses

# 查询购买单品价格大于100的用户的姓名和性别
select name, sex
	from user
	where id in (select user_id from commodity where price > 100);
+--------+------+
| name   | sex  |
+--------+------+
| 铁子   | m    |
| 潘子   | m    |
+--------+------+
2 rows in set (0.00 sec)

Generate subtable

# 查询购买总价大于1000的人
select name as 姓名,sex as 性别 
from user 
where id in 
	(select user_id 
	from (select user_id,sum(price) total from commodity group by user_id) as newTable 
	where newTable.total > 100);
+--------+--------+
| 姓名   | 性别   |
+--------+--------+
| 铁子   | m      |
| 潘子   | m      |
+--------+--------+
2 rows in set (0.00 sec)