mysql 非等值连接
#案例：查询员工的工资和工资级别
SELECT
salary,grade_level
FROM
employees AS e,job_grades AS j
WHERE
salary BETWEEN j.`lowest_sal` AND j.`highest_sal` #e表的salary字段会和j表的每行字段挨个比较
#案例：查询员工的工资和工资级别
SELECT
salary,grade_level
FROM
employees AS e,job_grades AS j
WHERE
salary BETWEEN j.`lowest_sal` AND j.`highest_sal` #e表的salary字段会和j表的每行字段挨个比较