安全等于 <=>

# 案例一

SELECT

            employees.last_name,

            employees.commission_pct

FROM

            employees

WHERE

            commission_pct <=> null;

# 案例二

SELECT

            employees.last_name,

            employees.salary

FROM

            employees

WHERE

            salary <=> 12000;

  # 安全等于 <=> 与 IS NULL的比较
  <=> 既可以判断数值也可以判断是否为NULL
  IS NULL 和 IS NOT NULL 只能判断NULL值