17.MySQL中的IS NULL 和 IS NOT NULL

1.准備

CREATE DATABASE mahaiwuji;
USE mahaiwuji;

CREATE TABLE emp
(
    empno INT PRIMARY KEY,
    ename VARCHAR(10),
    sal INT
) ENGINE = INNODB DEFAULT CHARSET = utf8;

INSERT INTO emp VALUES (1,'smith',2800);
INSERT INTO emp VALUES (2,'allen',NULL);
INSERT INTO emp VALUES (3,'ward',NULL);
INSERT INTO emp VALUES (4,'jones',1300);
INSERT INTO emp VALUES (5,'martin',NULL);
INSERT INTO emp VALUES (6,'blake',3000);

2.IS NULL

比較一個數據是否是NULL

-- 查找sal為NULL的數據
SELECT * FROM emp WHERE sal <=> NULL;
-- 等價於
SELECT * FROM emp WHERE sal IS NULL;

3.IS NOT NULL

比較一個數據是否不是NULL

-- 查找sal不為NULL的數據
SELECT * FROM emp WHERE sal IS NOT NULL;