17.MySQL中的IS NULL 和 IS NOT NULL

 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);

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;

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