# **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：

```text
non_subquery_operand comparison_operator (subquery)
```

其中，`comparison_operator` 指以下操作符：

```text
=  >  <  >=  <=  <>  !=  <=>
```

## **语法结构**

```sql
SELECT column_name(s) FROM table_name WHERE 'a' = (SELECT column1 FROM t1)
```

## **示例**

```text
create table t1 (a int);
create table t2 (a int, b int);
create table t3 (a int);
create table t4 (a int not null, b int not null);
insert into t1 values (2);
insert into t2 values (1,7),(2,7);
insert into t4 values (4,8),(3,8),(5,9);
insert into t3 values (6),(7),(3);

mysql> select * from t3 where a = (select b from t2);
ERROR 1105 (HY000): scalar subquery returns more than 1 row
mysql> select * from t3 where a = (select distinct b from t2);
+------+
| a    |
+------+
|    7 |
+------+
1 rows in set (0.01 sec)

mysql> select a,b from t4 where a > ( select a ,b from t2 where a>1);
ERROR 1105 (HY000): Internal error: Unknow type TUPLE
```