MySQL子查询详解(单行子查询、多行子查询与相关子查询)

# 法一:效率低 SELECT last_name,salary FROM employees WHERE last_name = 'ABEL' SELECT last_name,salary FROM employees WHERE salary > 11000; #法二：自连接 SELECT e2.last_name,e2.salary FROM employees e1,employees e2 WHERE e2.`salary` > e1.`salary` AND e1.`last_name` = 'Abel'; #法三：子查询 SELECT last_name,salary FROM employees WHERE salary > ( SELECT salary FROM employees WHERE last_name = 'ABEL' ); # 称谓的规范:外查询(主查询),内查询(子查询)

# 查询工资大于149号员工工资的信息 SELECT salary,last_name,employee_id FROM employees WHERE salary > ( SELECT salary FROM employees WHERE employee_id = 149 ); # 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 ); # 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 ); # 查询与141号员工的manager_id和department_id相同的其他员工的employee_id， # manager_id，department_id SELECT employee_id,manager_id,department_id FROM employees WHERE manager_id = (SELECT manager_id FROM employees WHERE employee_id = 141) AND department_id =(SELECT department_id FROM employees WHERE employee_id = 141) AND employee_id <> 141; #方式二：成对查询 SELECT employee_id,manager_id,department_id FROM employees WHERE (manager_id,department_id) = ( SELECT manager_id,department_id FROM employees WHERE employee_id = 141 ) AND employee_id <> 141; # 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 MIN(salary),department_id,salary FROM employees GROUP BY department_id HAVING MIN(salary) > ( SELECT MIN(salary) FROM employees WHERE department_id = 50 ) ORDER BY MIN(salary) DESC; # 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 FROM employees WHERE employee_id NOT IN ( SELECT manager_id FROM employees ); 

SELECT employee_id, last_name FROM employees WHERE salary = (SELECT MIN(salary) FROM employees GROUP BY department_id); 

SELECT employee_id, last_name FROM employees WHERE salary in (SELECT MIN(salary) FROM employees GROUP BY department_id); 

#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 SELECT employee_id,last_name,job_id,salary FROM employees WHERE salary  'IT_PROG'; #返回其它job_id中比job_id为‘IT_PROG'部门所有工资低的员工的员工号、姓名、job_id 以及salary SELECT employee_id,last_name,job_id,salary FROM employees WHERE salary  'IT_PROG'; 

# 聚合函数不能嵌套，单行函数才能嵌套使用 SELECT employee_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 )t_dept_avg_sal); #方式二 SELECT employee_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 #where manager_id is not null );

#回顾：查询员工中工资大于本公司平均工资的员工的last_name,salary和其department_id SELECT last_name,salary,department_id FROM employees e1 WHERE salary > ( SELECT AVG(salary) FROM employees e2 ); #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 SELECT last_name,salary,department_id FROM employees e1 WHERE salary > ( SELECT AVG(salary) FROM employees e2 WHERE department_id = e1.`department_id` ); #方式二,在from中声明子查询 SELECT e.last_name,e.salary,e.department_id FROM employees e,(SELECT department_id,AVG(salary) avg_sal FROM employees GROUP BY department_id) temp WHERE e.department_id = temp.department_id AND e.salary > temp.avg_sal; 

SELECT employee_id,salary FROM employees e1 ORDER BY ( SELECT department_name FROM departments d WHERE e1.`department_id` = d.`department_id` ); 

#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 employee_id,last_name,job_id FROM employees e WHERE  2 <= (SELECT COUNT(*) FROM job_history j WHERE e.`employee_id` = j.`employee_id` ); 

#方式一：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` #方式二：子查询 SELECT DISTINCT manager_id FROM employees SELECT employee_id,last_name,job_id,department_id FROM employees WHERE employee_id IN ( SELECT DISTINCT manager_id FROM employees ) #方式三：exists SELECT employee_id,last_name,job_id,department_id FROM employees e1 WHERE EXISTS ( SELECT * FROM employees e2 WHERE e1.`employee_id`= e2.`manager_id` )

 #方式一： SELECT d.department_id,d.department_name FROM employees e RIGHT JOIN departments d ON e.department_id = d.department_id WHERE e.department_id IS NULL; #方式二： SELECT department_id,department_name FROM departments d WHERE NOT EXISTS ( SELECT * FROM employees e WHERE d.`department_id` = e.`department_id` );

UPDATE table1 alias1 SET column = (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column); 

# 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); 

DELETE FROM employees e WHERE employee_id in (SELECT employee_id FROM emp_history WHERE employee_id = e.employee_id); 

   #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`  ( SELECT salary FROM employees WHERE last_name = 'Abel' );