MySQL | Subqueries

子查询

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.
The outer query is also called the main query,Inner queries are also called subqueries.

子查询分类

1. Sort by the results of the subquery
   • The query result is one：单行子查询;
   • There are multiple query results：多行子查询.
2. Sort by the number of times the subquery was executed
   • 子查询只执行一次,The execution result is executed as a condition of the main query：不相关子查询
   • The subquery is executed multiple times in a loop,依赖外部查询,To use an external table：相关子查询

OR(或) 与 IN效果一样

WHERE A=1 OR A=2

IS EQUAL TO

WHERE A IN (1, 2)

推荐IN

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 employee
GROUP BY department
HAVING MIN(salary) >  (SELECT MIN(salary)
								FROM employee
								WHERE department_id = 50)

CASE

题目：显示员工的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 employee
										WHERE location_id=1800), 
								THEN location='Canana'
								ELSE location='USA' END) location
FROM employee

ANY

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

SELECT employee_id, name, job_id, salary
FROM employees
WHERE job_id <> 'IT_PROG'
AND salary < ANY(SELECT salary
				 FROM employees
				 HAVING job_id='IT_PROG');

MIN 与 <=ALL效果一样

查询平均工资最低的部门id

Mysql中,聚合函数不可以嵌套使用.oracle可以.
MIN(AVG(SALARY))在mysqlkind of yes.

SELECT department_id
FROM employees
GROUP BY department
HAVING AVG(salary) <=	ALL(SELECT AVG(salary) avg_salary
					FROM employees
					GROUP BY department) 

相关子查询

如果存在,直接返回,不再查找.
如果不存在,继续查找,until found or all found.

EXIST：题目字眼“存在”
NOT EXIST：题目字眼“不存在”

自连接：DISTINCT + 自连接
自连接优于子查询,Self-connection is faster to process！