SQL 左連接
SQL LEFT JOIN 關鍵字
LEFT JOIN 關鍵字從左表（table1）傳回所有的行，即使右表（table2）中沒有符合。如果右表中沒有匹配，則結果為 NULL。
SQL LEFT JOIN 語法
SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;
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;
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=#table2 .column_name;
註記：在某些資料庫中，LEFT JOIN 稱為LEFT OUTER JOIN。
示範資料庫
在本教學中，我們將使用 php 樣本資料庫。
下面是選自"Websites" 表的資料：
+----+--------------+--- ------------------------+-------+---------+
| id | name | url | alexa | country |
+----+----------+------------------------------------ --------+-------+---------+
| 1 | Google | https://www.google.cm/ | 1 | USA | 1 |
| 2 | 淘寶 | https://www.taobao.com/ | 13 | CN cn/ | 4689 | CN |
| 4 | 微博 | http://weibo.com/ | 201 https://www.facebook.com/ | 3 | USA |
| 7 | stackoverflow | http://stackoverflow.com/ | 0 | IND |
+----+---------------+----------- ------------------+-------+---------+
#
SQL LEFT JOIN 實例下面的SQL 語句將會傳回所有網站及他們的訪問量（如果有的話）。 以下實例中我們把 Websites 當左表，access_log 作為右表：
#| id | name | url | alexa | country |
+----+----------+------------------------------------ --------+-------+---------+
| 1 | Google | https://www.google.cm/ | 1 | USA | 1 |
| 2 | 淘寶 | https://www.taobao.com/ | 13 | CN cn/ | 4689 | CN |
| 4 | 微博 | http://weibo.com/ | 201 https://www.facebook.com/ | 3 | USA |
| 7 | stackoverflow | http://stackoverflow.com/ | 0 | IND |
+----+---------------+----------- ------------------+-------+---------+
#
以下是"access_log" 網站存取記錄表的資料：
mysql> SELECT * FROM access_log;
+-----+--------- +-------+------------+
| aid | site_id | count | date |
+-----+------ ---+-------+------------+
| 1 | 1 | 45 | 2016-05-10 |#1 | 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 |
| -15 |
| 8 | 5 | 545 | 2016-05 -16 |
| 9 | 3 | 201 | 2016-05-17 |
+-----+---------+-------+----- ---------+
9 rows in set (0.00 sec)
+-----+--------- +-------+------------+
| aid | site_id | count | date |
+-----+------ ---+-------+------------+
| 1 | 1 | 45 | 2016-05-10 |#1 | 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 |
| -15 |
| 8 | 5 | 545 | 2016-05 -16 |
| 9 | 3 | 201 | 2016-05-17 |
+-----+---------+-------+----- ---------+
9 rows in set (0.00 sec)
SQL LEFT JOIN 實例下面的SQL 語句將會傳回所有網站及他們的訪問量（如果有的話）。 以下實例中我們把 Websites 當左表，access_log 作為右表：
實例
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;
執行上述SQL 輸出結果如下：LEFT JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count DESC;
註解：LEFT JOIN 關鍵字從左表（Websites）傳回所有的行，即使右表（access_log）中沒有符合。
