mysql> select * from test;+----+------------+-------+-----------+| id | name       | score | subject   |+----+------------+-------+-----------+|  1 | xiaoming   |    89 | shuxue    ||  2 | xiaohong   |    89 | shuxue    ||  3 | xiaohong   |    80 | english   ||  4 | xiaohong   |    80 | physics   ||  5 | xiaohong   |    80 | astronaut ||  6 | xiaoming   |    80 | physics   ||  7 | xiaoming   |    80 | astronaut ||  8 | xiaoming   |    80 | english   ||  9 | xiaobai    |    80 | astronaut || 10 | 1.2xiaobai |    80 | english   || 11 | 2.2xiaobai |    80 | physics   || 12 | 3xiaobai   |    80 | shuxue    || 13 | 123xiaohei |    80 | astronaut || 14 | xiaohei    |    80 | shuxue    || 15 | xiaohei    |    80 | physics   || 16 | .12xiaohei |    80 | english   |+----+------------+-------+-----------+16 rows in set (0.00 sec)mysql> select * from user;+------+----------+-----------+| id   | name     | address   |+------+----------+-----------+|    1 | xiaoming | beijing   ||    2 | xiaobai  | shandong  ||    3 | xiaohong | suzhou    ||    4 | xiaohei  | changchun |+------+----------+-----------+4 rows in set (0.00 sec)mysql> select *,(select address from user where user.name=test.name)as addr from test;+----+------------+-------+-----------+-----------+| id | name       | score | subject   | addr      |+----+------------+-------+-----------+-----------+|  1 | xiaoming   |    89 | shuxue    | beijing   ||  2 | xiaohong   |    89 | shuxue    | suzhou    ||  3 | xiaohong   |    80 | english   | suzhou    ||  4 | xiaohong   |    80 | physics   | suzhou    ||  5 | xiaohong   |    80 | astronaut | suzhou    ||  6 | xiaoming   |    80 | physics   | beijing   ||  7 | xiaoming   |    80 | astronaut | beijing   ||  8 | xiaoming   |    80 | english   | beijing   ||  9 | xiaobai    |    80 | astronaut | shandong  || 10 | 1.2xiaobai |    80 | english   | NULL      || 11 | 2.2xiaobai |    80 | physics   | NULL      || 12 | 3xiaobai   |    80 | shuxue    | NULL      || 13 | 123xiaohei |    80 | astronaut | NULL      || 14 | xiaohei    |    80 | shuxue    | changchun || 15 | xiaohei    |    80 | physics   | changchun || 16 | .12xiaohei |    80 | english   | NULL      |+----+------------+-------+-----------+-----------+16 rows in set (0.00 sec)mysql> select * from user where user.name in (select name from test where subject="shuxue");+------+----------+-----------+| id   | name     | address   |+------+----------+-----------+|    1 | xiaoming | beijing   ||    3 | xiaohong | suzhou    ||    4 | xiaohei  | changchun |+------+----------+-----------+3 rows in set (0.00 sec)