[17]SQL LEFT JOIN 關鍵字
阿新 • • 發佈:2017-11-11
網站 over count prev uno code www left country
[17]SQL LEFT JOIN 關鍵字
SQL LEFT JOIN 關鍵字
LEFT JOIN 關鍵字從左表(table1)返回所有的行,即使右表(table2)中沒有匹配。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;
或:
SELECT column_name(s) FROM table1 LEFT OUTER JOIN table2 ON table1.column_name=table2.column_name;
註釋:在某些數據庫中,LEFT JOIN 稱為 LEFT OUTER JOIN。
下面是選自 "Websites" 表的數據:
+----+--------------+---------------------------+-------+---------+
| id | name | url | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1 | Google | https://www.google.cm/ | 1 | USA |
| 2 | 淘寶 | https://www.taobao.com/ | 13 | CN |
| 3 | 菜鳥教程 | http://www.runoob.com/ | 4689 | CN |
| 4 | 微博 | http://weibo.com/ | 20 | CN |
| 5 | Facebook | https://www.facebook.com/ | 3 | USA |
| 7 | stackoverflow | http://stackoverflow.com/ | 0 | IND |
+----+---------------+---------------------------+-------+---------+
下面是 "access_log" 網站訪問記錄表的數據:
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 實例
下面的 SQL 語句將返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 作為左表,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;
執行以上 SQL 輸出結果如下:
註釋:LEFT JOIN 關鍵字從左表(Websites)返回所有的行,即使右表(access_log)中沒有匹配。
[17]SQL LEFT JOIN 關鍵字