#(2)非等值连接
#查询员工的工资级别
SELECT salary,grade_level
FROM employees e
JOIN job_grades g
ON e.salary BETWEEN g.lowest_sal AND g.highest_sal;
#查询每个工资级别的个数>20的个数，并且按工资级别降序
SELECT COUNT(*),grade_level
FROM employees e
JOIN job_grades g
ON e.salary BETWEEN g.lowest_sal AND g.highest_sal
GROUP BY grade_level
HAVING COUNT(*)>20
ORDER BY grade_level DESC;
#(3)自连接
#查询姓名中包含字符k的员工的名字、上级的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`=m.`employee_id`
WHERE e.`last_name` LIKE '%k%';
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