mysql三表关联查询练习

三张表：

1：公司表        

2、员工表

 3、中间关联表

 

-- 关联查询方式查询张飞在哪个公司
SELECT
	company.company_name AS '张飞公司' 
FROM
	company
	INNER JOIN com_emp ON com_emp.company_id = company.company_id
	INNER JOIN emp ON emp.emp_id = com_emp.emp_id 
WHERE
	emp.emp_name = '张飞';-- 	 子查询查询方式查询张飞在哪个公司
SELECT
	company.company_name 
FROM
	company 
WHERE
	company.company_id = ( SELECT com_emp.company_id FROM emp INNER JOIN com_emp ON emp.emp_name = '张飞' WHERE emp.emp_id = com_emp.emp_id ) -- 	-----------------------------------------------------------------------------------------------------------------------------------------------------------------------
-- 	查询建投公司的所有43岁以下员工的名字。
SELECT
	emp.emp_name 
FROM
	company
	INNER JOIN com_emp ON company.company_id = com_emp.company_id
	INNER JOIN emp ON com_emp.emp_id = emp.emp_id 
WHERE
	emp.`em_age` < '43' 
	AND company.company_name = '建投' ;
	
	-------------------------------------------------------------------------- 内连接等价于：
SELECT
	* 
FROM
	company,
	com_emp,
	emp 
WHERE
	company.company_id = com_emp.company_id 
	AND com_emp.emp_id = emp.emp_id 
	AND emp.`em_age` < '43' 
	AND company.company_name = '建投' 
	
	
	-- --------------------------------------------	子查询实现查询建投公司的所有43岁以下员工的名字

SELECT emp.emp_name from emp where emp.emp_id=(
SELECT
	com_emp.emp_id
FROM
	company,
	com_emp 
WHERE
	company.company_id = com_emp.company_id 
	AND company.company_name = '建投')