# **IS**

## **语法说明**

`IS` 运算符用于测试数值是否为布尔值，若是布尔值，则返回结果为 `true`。其中 `boolean_value` 可以为 `TRUE`、`FALSE` 或 `UNKNOWN`。

## **语法结构**

```text
> IS boolean_value
```

## **示例**

```text
mysql> SELECT 1 IS TRUE, 0 IS FALSE, NULL IS UNKNOWN;
+-----------+------------+-----------------+
| 1 is true | 0 is false | null is unknown |
+-----------+------------+-----------------+
| true      | true       | true            |
+-----------+------------+-----------------+
1 row in set (0.01 sec)
```

```text
create table t1 (a boolean,b bool);
insert into t1 values (0,1),(true,false),(true,1),(0,false),(NULL,NULL);

mysql> select * from t1;
+-------+-------+
| a     | b     |
+-------+-------+
| false | true  |
| true  | false |
| true  | true  |
| false | false |
| NULL  | NULL  |
+-------+-------+
mysql> select * from t1 where a<=b and a is not NULL;
+-------+-------+
| a     | b     |
+-------+-------+
| false | true  |
| true  | true  |
| false | false |
+-------+-------+
3 rows in set (0.01 sec)
```