17.MySQL中的IS NULL 和 IS NOT NULL

Posted 2021-03-02 码海无际

1.准备

 1 CREATE DATABASE mahaiwuji;
 2 USE mahaiwuji;
 3 
 4 CREATE TABLE emp
 5 (
 6     empno INT PRIMARY KEY,
 7     ename VARCHAR(10),
 8     sal INT
 9 ) ENGINE = INNODB DEFAULT CHARSET = utf8;
10 
11 INSERT INTO emp VALUES (1,\'smith\',2800);
12 INSERT INTO emp VALUES (2,\'allen\',NULL);
13 INSERT INTO emp VALUES (3,\'ward\',NULL);
14 INSERT INTO emp VALUES (4,\'jones\',1300);
15 INSERT INTO emp VALUES (5,\'martin\',NULL);
16 INSERT INTO emp VALUES (6,\'blake\',3000);

2.IS NULL

比较一个数据是否是NULL

1 -- 查找sal为NULL的数据
2 SELECT * FROM emp WHERE sal <=> NULL;
3 -- 等价于
4 SELECT * FROM emp WHERE sal IS NULL;

3.IS NOT NULL

比较一个数据是否不是NULL

1 -- 查找sal不为NULL的数据
2 SELECT * FROM emp WHERE sal IS NOT NULL;