子查询与比较操作符的使用

语法描述

子查询与比较操作符最常见的用法如下：

  1. non_subquery_operand comparison_operator (subquery)

其中，comparison_operator 指以下操作符：

  1. =  >  <  >=  <=  <>  !=  <=>

语法结构

  1. > SELECT column_name(s) FROM table_name WHERE 'a' = (SELECT column1 FROM t1)

示例

  1. create table t1 (a int);
  2. create table t2 (a int, b int);
  3. create table t3 (a int);
  4. create table t4 (a int not null, b int not null);
  5. insert into t1 values (2);
  6. insert into t2 values (1,7),(2,7);
  7. insert into t4 values (4,8),(3,8),(5,9);
  8. insert into t3 values (6),(7),(3);
  10. mysql> select * from t3 where a = (select b from t2);
  11. ERROR 1105 (HY000): scalar subquery returns more than 1 row
  12. mysql> select * from t3 where a = (select distinct b from t2);
  13. +------+
  14. | a    |
  15. +------+
  16. |    7 |
  17. +------+
  18. 1 rows in set (0.01 sec)
  20. mysql> select a,b from t4 where a > ( select a ,b from t2 where a>1);
  21. ERROR 1105 (HY000): Internal error: Unknow type TUPLE