#案例:查询没有奖金的员工名和奖金率
#=或<>号不能判断是否等于null值,用is和is not判断
SELECT
last_name,
commission_pct
FROM
employees
WHERE
commission_pct IS NULL;
#案例:查询有奖金
SELECT
last_name,
commission_pct
FROM
employees
WHERE
commission_pct IS NOT NULL;
安全等于: <=>,也可以判断null值和普通类型的值
SELECT
last_name,
commission_pct
FROM
employees
WHERE
commission_pct <=> NULL;