子查询:一条查询语句中又嵌套了另一条完整的select语句,其中被嵌套的select语句,称为子查询或内查询在外面的查询语句,称为主查询或外查询;
行子查询:子查询返回的结果集是一行多列,该子查询的结果通常是对表的某行数据进行查询而返回的结果集;(单行多列)
列子查询:子查询返回的结果集是多行一列,该结果通常来自对表的某个字段查询返回;(多行)
标量子查询:标量子查询是指子查询返回的是单一值的标量,如一个数字或一个字符串,也是子查询中最简单的返回形式;(单行)
表子查询:子查询返回的结果集一般是多行多列
- 按子查询出现的位置分类:
SELECT 后面:
仅仅支持标量子查询
例子如下:
#案例:查询每个部门的员工个数 SELECT d.*,( SELECT COUNT(*) FROM employees e WHERE e.department_id = d.`department_id` ) 个数 FROM departments d; #案例2:查询员工号=102的部门名 SELECT ( SELECT department_name FROM departments d INNER JOIN employees e ON d.department_id=e.department_id WHERE e.employee_id=102 ) 部门名;
FROM 后面:
支持表子查询,将子查询结果充当一张表,要求必须起别名
例子如下:
#案例:查询每个部门的平均工资的工资等级 #①查询每个部门的平均工资 SELECT AVG(salary),department_id FROM employees GROUP BY department_id SELECT * FROM job_grades; #②连接①的结果集和job_grades表,筛选条件平均工资 between lowest_sal and highest_sal SELECT ag_dep.*,g.`grade_level` FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id ) ag_dep INNER JOIN job_grades g ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;
WHERE 或 HAVING 后面:
标量子查询
列子查询
行子查询
特点:
①子查询放在小括号内
②子查询一般放在条件的右侧
③标量子查询,一般搭配着单行操作符使用:> ,< ,>= ,<= ,= ,<>
列子查询,一般搭配着多行操作符使用:IN,ANY/SOME,ALL
④子查询的执行优先于主查询执行,主查询的条件用到了子查询的结果
举例如下:
#1.标量子查询 #案例1:谁的工资比 Abel 高? #①查询Abel的工资 SELECT salary FROM employees WHERE last_name = 'Abel' #②查询员工的信息,满足 salary>①结果 SELECT * FROM employees WHERE salary>( SELECT salary FROM employees WHERE last_name = 'Abel' ); #案例2:返回job_id与141号员工相同,salary比143号员工多的员工 姓名,job_id 和工资 #①查询141号员工的job_id SELECT job_id FROM employees WHERE employee_id = 141 #②查询143号员工的salary SELECT salary FROM employees WHERE employee_id = 143 #③查询员工的姓名,job_id 和工资,要求job_id=①并且salary>② SELECT last_name,job_id,salary FROM employees WHERE job_id = ( SELECT job_id FROM employees WHERE employee_id = 141 ) AND salary>( SELECT salary FROM employees WHERE employee_id = 143 ); #案例3:返回公司工资最少的员工的last_name,job_id和salary #①查询公司的 最低工资 SELECT MIN(salary) FROM employees #②查询last_name,job_id和salary,要求salary=① SELECT last_name,job_id,salary FROM employees WHERE salary=( SELECT MIN(salary) FROM employees ); #案例4:查询最低工资大于50号部门最低工资的部门id和其最低工资 #①查询50号部门的最低工资 SELECT MIN(salary) FROM employees WHERE department_id = 50 #②查询每个部门的最低工资 SELECT MIN(salary),department_id FROM employees GROUP BY department_id #③ 在②基础上筛选,满足min(salary)>① SELECT MIN(salary),department_id FROM employees GROUP BY department_id HAVING MIN(salary)>( SELECT MIN(salary) FROM employees WHERE department_id = 50 ); #非法使用标量子查询 SELECT MIN(salary),department_id FROM employees GROUP BY department_id HAVING MIN(salary)>( SELECT salary FROM employees WHERE department_id = 250 ); #2.列子查询(多行子查询) #案例1:返回location_id是1400或1700的部门中的所有员工姓名 #①查询location_id是1400或1700的部门编号 SELECT DISTINCT department_id FROM departments WHERE location_id IN(1400,1700) #②查询员工姓名,要求部门号是①列表中的某一个 SELECT last_name FROM employees WHERE department_id <>ALL( SELECT DISTINCT department_id FROM departments WHERE location_id IN(1400,1700) ); #案例2:返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工的员工号、姓名、job_id 以及salary #①查询job_id为‘IT_PROG’部门任一工资 SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG' #②查询员工号、姓名、job_id 以及salary,salary<(①)的任意一个 SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<ANY( SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG' ) AND job_id<>'IT_PROG'; #或 SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<( SELECT MAX(salary) FROM employees WHERE job_id = 'IT_PROG' ) AND job_id<>'IT_PROG'; #案例3:返回其它部门中比job_id为‘IT_PROG’部门所有工资都低的员工 的员工号、姓名、job_id 以及salary SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<ALL( SELECT DISTINCT salary FROM employees WHERE job_id = 'IT_PROG' ) AND job_id<>'IT_PROG'; #或 SELECT last_name,employee_id,job_id,salary FROM employees WHERE salary<( SELECT MIN( salary) FROM employees WHERE job_id = 'IT_PROG' ) AND job_id<>'IT_PROG'; #3、行子查询(结果集一行多列或多行多列) #案例:查询员工编号最小并且工资最高的员工信息 SELECT * FROM employees WHERE (employee_id,salary)=( SELECT MIN(employee_id),MAX(salary) FROM employees ); #①查询最小的员工编号 SELECT MIN(employee_id) FROM employees #②查询最高工资 SELECT MAX(salary) FROM employees #③查询员工信息 SELECT * FROM employees WHERE employee_id=( SELECT MIN(employee_id) FROM employees )AND salary=( SELECT MAX(salary) FROM employees );
EXISTS 后面(相关子查询)
表子查询(一般是多行多列)
举例如下:
#IN SELECT department_name FROM departments d WHERE d.`department_id` IN( SELECT department_id FROM employees ) #EXISTS SELECT department_name FROM departments d WHERE EXISTS( SELECT * FROM employees e WHERE d.`department_id`=e.`department_id` );