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SQL99语法实现多表查询

3.SQL99语法实现多表查询

3.1 内连接(INNER JOIN)的实现

语法

SELECT 字段列表
FROM A表 INNER JOIN B表
ON 关联条件
WHERE 等其他子句;

SELECT e.employee_id, e.last_name, e.department_id,
d.department_id, d.location_id
FROM employees e JOIN departments d
ON (e.department_id = d.department_id);

3.2 外连接(OUTER JOIN)的实现
3.2.1.左外连接

语法

#实现查询结果是A
SELECT 字段列表
FROM A表 LEFT JOIN B表
ON 关联条件
WHERE 等其他子句;
例
SELECT e.last_name, e.department_id, d.department_name
FROM employees e
LEFT OUTER JOIN departments d
ON (e.department_id = d.department_id) ;
3.2.2.右外连接
SELECT 字段列表
FROM A表 RIGHT JOIN B表
ON 关联条件
WHERE 等其他子句;

例
SELECT e.last_name, e.department_id, d.department_name
FROM employees e
RIGHT OUTER JOIN departments d
ON (e.department_id = d.department_id) ;

3.2.3.满外连接
  • 满外连接的结果 = 左右表匹配的数据 + 左表没有匹配到的数据 + 右表没有匹配到的数据。
  • SQL99是支持满外连接的。使用FULL JOIN 或 FULL OUTER JOIN来实现。
  • 需要注意的是,MySQL不支持FULL JOIN,但是可以用 LEFT JOIN UNION RIGHT join代替。

4.UNION的使用

合并查询结果 利用UNION关键字,可以给出多条SELECT语句,并将它们的结果组合成单个结果集。合并 时,两个表对应的列数和数据类型必须相同,并且相互对应。各个SELECT语句之间使用UNION或UNION ALL关键字分隔

语法

SELECT column,... FROM table1
UNION [ALL]
SELECT column,... FROM table2

image.png image.png

注意:执行UNION ALL语句时所需要的资源比UNION语句少。如果明确知道合并数据后的结果数据不存在重复数据,或者不需要去除重复的数据,则尽量使用UNION ALL语句,以提高数据查询的效率

5种SQL JOINS的实现

image.png

代码实现

#中图:内连接 A∩B
SELECT employee_id,last_name,department_name
FROM employees e JOIN departments d
ON e.`department_id` = d.`department_id`;
#左上图:左外连接
SELECT employee_id,last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`;
#右上图:右外连接
SELECT employee_id,last_name,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`;
#左中图:A - A∩B
SELECT employee_id,last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL

#右中图:B-A∩B
SELECT employee_id,last_name,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL
#右下图
#左中图 + 右中图 A ∪B- A∩B 或者 (A - A∩B) ∪ (B - A∩B)
SELECT employee_id,last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL
SELECT employee_id,last_name,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE e.`department_id` IS NULL
左下图
SELECT employee_id,last_name,department_name
FROM employees e LEFT JOIN departments d
ON e.`department_id` = d.`department_id`
WHERE d.`department_id` IS NULL
UNION ALL #没有去重操作,效率高
SELECT employee_id,last_name,department_name
FROM employees e RIGHT JOIN departments d
ON e.`department_id` = d.`department_id`;
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