关联子查询
关联子查询,是指子查询的执行依赖于外部查询,通常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。
- 查询员工工资大于本部门平均工资的员工的last_name、salary和department_id。
# 查询员工中工资大于本部门平均工资的员工的last_name,salary和department_id
# 方式1
SELECT last_name,salary,department_id
FROM employees e1
WHERE salary > (
SELECT AVG(salary)
FROM employees e2
WHERE e1.department_id = e2.department_id
GROUP BY department_id
); -- 返回38行记录
# 方式2
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
) t_dep_avg_sal
WHERE e.department_id = t_dep_avg_sal.department_id
AND e.salary > t_dep_avg_sal.avg_sal; -- 返回38行记录
- 查询员工的employee_id、salary,按照department_name排序。
# 查询员工的employee_id、salary,按照department_name排序
SELECT department_id,salary
FROM employees e
ORDER BY (
SELECT department_name
FROM departments d
WHERE e.department_id = d.department_id
); -- 返回107行记录
- 如果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job_id。
# 如果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
) -- 返回3行记录
关键字:EXISTS 和 NOT EXISTS
关联子查询,通常会和EXISTS
一起使用,用来检查子查询中是否有满足条件的行。
- 如果子查询中不存在满足条件的行,
EXISTS
会返回false
,并继续在子查询中查找。
- 如果子查询中存在满足条件的行,则停止查找,返回
true
。
- 查询公司管理者的employee_id、last_name、job_id、department_id。
# 查询公司管理者的employee_id、last_name、job_id、department_id
# 方式1:自连接
SELECT DISTINCT e1.employee_id,e1.last_name,e1.job_id,e1.department_id
FROM employees e1,employees e2
WHERE e1.employee_id = e2.manager_id;
# 方式2:子查询(多行子查询)
SELECT employee_id,last_name,job_id,department_id
FROM employees
WHERE employee_id IN (
SELECT DISTINCT manager_id
FROM employees
)
# 方式3:子查询(关联子查询)
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
)
- 查询departments表中,不存在于employees表中的部门的department_id和department_name。
# 查询departments表中,不存在于employees表中的部门的department_id和department_name
# 方式1;右外连接
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; -- 返回16行记录
# 方式2:关联子查询(NOT EXISTS)
SELECT department_id,department_name
FROM departments d
WHERE NOT EXISTS (
SELECT DISTINCT department_id
FROM employees e
WHERE d.department_id = e.department_id
); -- 返回16行记录
相关链接
MySQL的子查询