详解SQL嵌套子查询

2024-08-31 00:46:50

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一些初级程序员常常对SQL语法中的子查询，由其对嵌套子查询（子查询中包含一个子查询）的使用比较生疏，本文就此做一个基本讲解，相信新手会有一定收获。
    使用子查询的原则
1.一个子查询必须放在圆括号中。
2.将子查询放在比较条件的右边以增加可读性。
子查询不包含 ORDER BY 子句。对一个 SELECT 语句只能用一个 ORDER BY 子句,
并且如果指定了它就必须放在主 SELECT 语句的最后。
ORDER BY 子句可以使用,并且在进行 Top-N 分析时是必须的。
3.在子查询中可以使用两种比较条件：单行运算符和多行运算符。

    子查询的类型
单行子查询：从内 SELECT 语句只返回一行的查询
多行子查询：从内 SELECT 语句返回多行的查询

    单行子查询
单行子查询是从内查询返回一行的查询。在该子查询类型中用一个单行操作符。幻灯片中列出了单行操作符。
例

显示那些 job ID 与雇员 141 相同的雇员。
SELECT last_name, job_id
FROM employees
WHERE job_id =
(SELECT job_id
FROM employees
WHERE employee_id = 141);

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);

显示 job ID 与雇员 141 相同,并且薪水高于雇员 143 的那些雇员。

注：外和内查询可以从不同的表中取得数据。

SELECT last_name, job_id, salary
FROM employees
WHERE salary =
(SELECT MIN(salary)
FROM employees);

求所有人谁的工资最小。

SELECT department_id, MIN(salary)
FROM employees
GROUP BY department_id
HAVING MIN(salary) >
(SELECT MIN(salary)
FROM employees
WHERE department_id = 50);

求每个部门的最小工资,但是要高于50号部门的工资。

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

问题出现在：单行子查询返回了多个查询值;
应改为：
SELECT employee_id, last_name
FROM employees
WHERE salary in
(SELECT MIN(salary)
FROM employees
GROUP BY department_id);

SELECT last_name, job_id
FROM employees
WHERE job_id =
(SELECT job_id
FROM employees
WHERE last_name = ’Haas’);

如果子查询返回的是零值,不会对主程序造成影响;
如果子查询返回的是空值,那么会影响主程序的返回值;
SELECT employee_id,
last_name
FROM employees
WHERE employee_id NOT IN
(SELECT manager_id
FROM employees)

SELECT employee_id,
last_name
FROM employees
WHERE employee_id NOT IN
(SELECT manager_id
FROM employees
WHERE manager_id IS NOT NULL)

放在select下的子查询的返回值必须是一个具体值,
from后面也可以加子查询;
having后面也可以加子查询;
order by后面也可以;

多列子查询适应于：成对比较;非成对比较。

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 (178,174))
AND employee_id NOT IN (178,174);

输出：176 149 80

只有要查询的东西和你子查询返回的东西一一对应上了,你的查询才能成功。
如果有一个对应不上那么你的查询不会成功。

非成对的子查询：
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);

输出：144 124 50
143 124 50
142 124 50
176 149 80
上面两个程序就是成对子查询和非成对子查询两者之间的区别。

如果我想去显示员工信息,要求：员工的工资高于本部门的平均工资。
SELECT a.last_name,
a.salary,
a.department_id
FROM employees a
WHERE a.salary >
(SELECT AVG(salary)
FROM employees b
WHERE b.department_id = a.department_id);

in line view(内联视图)

SELECT a.last_name, a.salary,
a.department_id, b.salavg
FROM employees a, (SELECT department_id,
AVG(salary) salavg
FROM employees
GROUP BY department_id) b
WHERE a.department_id = b.department_id
AND a.salary > b.salavg;

数据分散于多个表中需要进行拼合
SELECT DISTINCT c.LastName, c.FirstName
FROM Person.Contact c JOIN HumanResources.Employee e
ON e.ContactID = c.ContactID WHERE EmployeeID IN
(SELECT SalesPersonID
FROM Sales.SalesOrderHeader
WHERE SalesOrderID IN
(SELECT SalesOrderID
FROM Sales.SalesOrderDetail
WHERE ProductID IN
(SELECT ProductID
FROM Production.Product p
WHERE ProductNumber LIKE 'FW123%')));