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一.Join语法概述
join 用于多表中字段之间的联系,语法如下:
... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON conditiona
table1:左表;table2:右表。
JOIN 按照功能大致分为如下三类:
INNER JOIN(内连接,或等值连接):取得两个表中存在连接匹配关系的记录。
LEFT JOIN(左连接):取得左表(table1)完全记录,即是右表(table2)并无对应匹配记录。
RIGHT JOIN(右连接):与 LEFT JOIN 相反,取得右表(table2)完全记录,即是左表(table1)并无匹配对应记录。
注意:mysql不支持Full join,不过可以通过UNION 关键字来合并 LEFT JOIN 与 RIGHT JOIN来模拟FULL join.
先看一下实验的两张表:
EXPLAIN SELECT * FROM comments gc JOIN comments_for gcf ON gc.comments_id=gcf.comments_id;
SELECT * FROM comments gc JOIN comments_for gcf ON gc.comments_id=gcf.comments_id WHERE gc.comments_id =2056
EXPLAIN SELECT * FROM comments gc JOIN comments_for gcf ON gc.order_id=gcf.product_id
EXPLAIN SELECT * FROM comments gc LEFT JOIN comments_for gcf ON gc.comments_id=gcf.comments_id
EXPLAIN SELECT * FROM comments_for gcf LEFT JOIN comments gc ON gc.comments_id=gcf.comments_id WHERE gcf.comments_id =2056
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