mysql 子查询
2021/7/11 19:09:44
本文主要是介绍mysql 子查询,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!
实例
SELECT * FROM student WHERE class_id IN ( SELECT id FROM class WHERE id = 2 );
+----+--------+------+------+----------+--------+ | id | name | sex | age | class_id | status | +----+--------+------+------+----------+--------+ | 4 | 小红 | 女 | 23 | 2 | 1 | | 5 | 小花 | 女 | 19 | 2 | 1 | +----+--------+------+------+----------+--------+ 2 rows in set (0.00 sec)
实例
select * from student where class_id < (select id from class where id = 2);
+----+--------+------+------+----------+--------+ | id | name | sex | age | class_id | status | +----+--------+------+------+----------+--------+ | 1 | 张三 | 男 | 18 | 1 | 1 | | 2 | 王五 | 男 | 20 | 1 | 1 | | 3 | 李四 | 男 | 20 | 1 | 1 | +----+--------+------+------+----------+--------+ 3 rows in set (0.00 sec)
实例
select * from student where class_id = (select id from class where id = 2);
+----+--------+------+------+----------+--------+ | id | name | sex | age | class_id | status | +----+--------+------+------+----------+--------+ | 4 | 小红 | 女 | 23 | 2 | 1 | | 5 | 小花 | 女 | 19 | 2 | 1 | +----+--------+------+------+----------+--------+ 2 rows in set (0.00 sec)
实例
SELECT class.`id`, class.`name`, ( SELECT count( * ) FROM student WHERE student.`class_id` = class.`id` ) AS 学生数 FROM class;
+------+---------+-----------+ | id | name | 学生数 | +------+---------+-----------+ | 1 | 班级1 | 3 | | 2 | 班级2 | 2 | | 3 | 班级3 | 2 | +------+---------+-----------+ 3 rows in set (0.00 sec)
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