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子查询指一个查询语句嵌套在另一个查询语句内部的查询,这个特性从MySQL 4.1开始引入。
SQL 中子查询的使用大大增强了 SELECT 查询的能力,因为很多时候查询需要从结果集中获取数据,或者需要从同一个表中先计算得出一个数据结果,然后与这个数据结果(可能是某个标量,也可能是某个集合)进行比较。
现有解决方式:
#方式一: SELECT salary FROM employees WHERE last_name = 'Abel'; SELECT last_name,salary FROM employees WHERE salary > 11000;
#方式二:自连接 SELECT e2.last_name,e2.salary FROM employees e1,employees e2 WHERE e1.last_name = 'Abel' AND e1.`salary` < e2.`salary`
#方式三:子查询 SELECT last_name,salary FROM employees WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'Abel' );
子查询的基本语法结构:
子查询(内查询)在主查询之前一次执行完成。
子查询的结果被主查询(外查询)使用 。
注意事项
1. 分类方式1:
我们按内查询的结果返回一条还是多条记录,将子查询分为单行子查询、多行子查询。
•单行子查询
•多行子查询
2. 分类方式2:
我们按内查询是否被执行多次,将子查询划分为相关(或关联)子查询和不相关(或非关联)子查询。
子查询从数据表中查询了数据结果,如果这个数据结果只执行一次,然后这个数据结果作为主查询的条件进行执行,那么这样的子查询叫做不相关子查询。
同样,如果子查询需要执行多次,即采用循环的方式,先从外部查询开始,每次都传入子查询进行查询,然后再将结果反馈给外部,这种嵌套的执行方式就称为相关子查询。
题目:查询工资大于149号员工工资的员工的信息
题目: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
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);
题目:返回公司工资最少的员工的last_name,job_id和salary
SELECT last_name, job_id, salary
FROM employees
WHERE salary =
(SELECT MIN(salary)
FROM employees);
题目:查询与141号或174号员工的manager_id和department_id相同的其他员工的employee_id,manager_id,department_id
实现方式1:不成对比较
SELECT employee_id, manager_id, department_id FROM employees WHERE manager_id IN (SELECT manager_id FROM employees WHERE employee_id IN (174,141)) AND department_id IN (SELECT department_id FROM employees WHERE employee_id IN (174,141)) AND employee_id NOT IN(174,141);
实现方式2:成对比较
SELECT employee_id, manager_id, department_id FROM employees WHERE (manager_id, department_id) IN (SELECT manager_id, department_id FROM employees WHERE employee_id IN (141,174)) AND employee_id NOT IN (141,174);
题目: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
SELECT department_id, MIN(salary) FROM employees GROUP BY department_id HAVING MIN(salary) > (SELECT MIN(salary) FROM employees WHERE department_id = 50);
在CASE表达式中使用单列子查询:
题目:显式员工的employee_id,last_name和location。其中,若员工department_id与location_id为1800的department_id相同,则location为’Canada’,其余则为’USA’。
SELECT employee_id, last_name, (CASE department_id WHEN (SELECT department_id FROM departments WHERE location_id = 1800) THEN 'Canada' ELSE 'USA' END) location FROM employees;
SELECT last_name, job_id FROM employees WHERE job_id = (SELECT job_id FROM employees WHERE last_name = 'Haas');
子查询不返回任何行
SELECT employee_id, last_name FROM employees WHERE salary = (SELECT MIN(salary) FROM employees GROUP BY department_id);
多行子查询使用单行比较符
体会 ANY 和 ALL 的区别
题目:返回其它job_id中比job_id为‘IT_PROG’部门任一工资低的员工的员工号、姓名、job_id 以及salary
题目:返回其它job_id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
题目:查询平均工资最低的部门id
#方式1: SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary) = ( SELECT MIN(avg_sal) FROM ( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id ) dept_avg_sal )
#方式2: SELECT department_id FROM employees GROUP BY department_id HAVING AVG(salary) <= ALL ( SELECT AVG(salary) avg_sal FROM employees GROUP BY department_id )
SELECT last_name FROM employees WHERE employee_id NOT IN ( SELECT manager_id FROM employees );
如果子查询的执行依赖于外部查询,通常情况下都是因为子查询中的表用到了外部的表,并进行了条件关联,因此每执行一次外部查询,子查询都要重新计算一次,这样的子查询就称之为关联子查询。
相关子查询按照一行接一行的顺序执行,主查询的每一行都执行一次子查询。
说明:子查询中使用主查询中的列
题目:查询员工中工资大于本部门平均工资的员工的last_name,salary和其department_id
1. 方式一:相关子查询
2. 方式二:在 FROM 中使用子查询
SELECT last_name,salary,e1.department_id FROM employees e1,(SELECT department_id,AVG(salary) dept_avg_sal FROM employees GROUP BY department_id) e2 WHERE e1.`department_id` = e2.department_id AND e2.dept_avg_sal < e1.`salary`;
from型的子查询:子查询是作为from的一部分,子查询要用()引起来,并且要给这个子查询取别名,把它当成一张“临时的虚拟的表”来使用。
在ORDER BY 中使用子查询:
题目:查询员工的id,salary,按照department_name 排序
SELECT employee_id,salary FROM employees e ORDER BY ( SELECT department_name FROM departments d WHERE e.`department_id` = d.`department_id` );
题目:若employees表中employee_id与job_history表中employee_id相同的数目不小于2,输出这些相同id的员工的employee_id,last_name和其job_id
SELECT e.employee_id, last_name,e.job_id FROM employees e WHERE 2 <= (SELECT COUNT(*) FROM job_history WHERE employee_id = e.employee_id);
题目:查询公司管理者的employee_id,last_name,job_id,department_id信息
方式一:
SELECT employee_id, last_name, job_id, department_id FROM employees e1 WHERE EXISTS ( SELECT * FROM employees e2 WHERE e2.manager_id = e1.employee_id);
方式二:自连接
SELECT DISTINCT e1.employee_id, e1.last_name, e1.job_id, e1.department_id FROM employees e1 JOIN employees e2 WHERE e1.employee_id = e2.manager_id;
方式三:
SELECT employee_id,last_name,job_id,department_id FROM employees WHERE employee_id IN ( SELECT DISTINCT manager_id FROM employees );
题目:查询departments表中,不存在于employees表中的部门的department_id和department_name
SELECT department_id, department_name FROM departments d WHERE NOT EXISTS (SELECT 'X' FROM employees WHERE department_id = d.department_id);
UPDATE table1 alias1 SET column = (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column);
使用相关子查询依据一个表中的数据更新另一个表的数据。
题目:在employees中增加一个department_name字段,数据为员工对应的部门名称
# 1) ALTER TABLE employees ADD(department_name VARCHAR2(14)); # 2) UPDATE employees e SET department_name = (SELECT department_name FROM departments d WHERE e.department_id = d.department_id);
DELETE FROM table1 alias1 WHERE column operator (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column);
使用相关子查询依据一个表中的数据删除另一个表的数据。
题目:删除表employees中,其与emp_history表皆有的数据
DELETE FROM employees e WHERE employee_id in (SELECT employee_id FROM emp_history WHERE employee_id = e.employee_id);
问题: 谁的工资比Abel的高?
解答:
#方式1:自连接 SELECT e2.last_name,e2.salary FROM employees e1,employees e2 WHERE e1.last_name = 'Abel' AND e1.`salary` < e2.`salary`
#方式2:子查询 SELECT last_name,salary FROM employees WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'Abel' );
问题: 以上两种方式有好坏之分吗?
解答: 自连接方式好!
题目中可以使用子查询,也可以使用自连接。一般情况建议你使用自连接,因为在许多 DBMS 的处理过程中,对于自连接的处理速度要比子查询快得多。
可以这样理解:子查询实际上是通过未知表进行查询后的条件判断,而自连接是通过已知的自身数据表进行条件判断,因此在大部分 DBMS 中都对自连接处理进行了优化。