SQL 教程-SQL LEFT JOIN 关键字
SQL LEFT JOIN 关键字LEFT JOIN 关键字从左表（table1）返回所有的行，即使右表（table2）中没有匹配。如果右表中没有匹配，则结果为 NULL。SQL LEFT JOIN 语法SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;
或：SELECT column_name(s)
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=table2.column_name;
注释：在某些数据库中，LEFT JOIN 称为 LEFT OUTER JOIN。https://www.runoob.com/wp-content/uploads/2013/09/img_leftjoin.gif
下面是 "access_log" 网站访问记录表的数据：mysql> SELECT * FROM access_log;+-----+---------+-------+------------+| aid | site_id | count | date |+-----+---------+-------+------------+| 1 | 1 | 45 | 2016-05-10 || 2 | 3 | 100 | 2016-05-13 || 3 | 1 | 230 | 2016-05-14 || 4 | 2 | 10 | 2016-05-14 || 5 | 5 | 205 | 2016-05-14 || 6 | 4 | 13 | 2016-05-15 || 7 | 3 | 220 | 2016-05-15 || 8 | 5 | 545 | 2016-05-16 || 9 | 3 | 201 | 2016-05-17 |+-----+---------+-------+------------+9 rows in set (0.00 sec)
SQL LEFT JOIN 实例下面的 SQL 语句将返回所有网站及他们的访问量（如果有的话）。以下实例中我们把 Websites 作为左表，access_log 作为右表：实例SELECT Websites.name, access_log.count, access_log.date
FROM Websites
LEFT JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count DESC;
执行以上 SQL 输出结果如下：https://www.runoob.com/wp-content/uploads/2013/09/left-join1.jpg
注释：LEFT JOIN 关键字从左表（Websites）返回所有的行，即使右表（access_log）中没有匹配。
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