mysql多表查询连接方式

内连接
  • 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;
  • 示范
    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);


    image.png
外连接(OUTER JOIN)
  • 左外连接
  • 语法
    A SELECT 字段列表 FROM A表 LEFT JOIN B表 ON 关联条件 WHERE 等其他子句;
  • 示范
    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) ;


    image.png

    左连接会把join左边的表左右数据都显示出来,右边表符合条件的显示出来

  • 右外连接
  • 语法
    FROM A表 RIGHT JOIN B表 ON 关联条件 WHERE 等其他子句;
  • 示范
    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) ;


    image.png
满外连接(FULL OUTER JOIN)

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。

union的使用
  • union
    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。
  • 语法格式
    SELECT column,... FROM table1 UNION [ALL] SELECT column,... FROM table2
    UNION 操作符返回两个查询的结果集的并集,去除重复记录。
  • UNION ALL
    UNION ALL操作符返回两个查询的结果集的并集。对于两个结果集的重复部分,不去重。
    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。
7种sql join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∩BSELECT 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

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