在实际运用中,我们常常会将多张表连接在一起进行查询,在一张表中使用外键跟另一张表进行关联。
比如有如下两张表:
mysql> SELECT * FROM Websites;
+----+--------------+---------------------------+-------+---------+
| id | name | url | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1 | Google | //www.google.cm/ | 1 | USA |
| 2 | 淘宝 | //www.taobao.com/ | 13 | CN |
| 3 | 菜鸟教程 | http://www.runoob.com/ | 4689 | CN |
| 4 | 微博 | http://weibo.com/ | 20 | CN |
| 5 | Facebook | //www.facebook.com/ | 3 | USA |
| 7 | stackoverflow | http://stackoverflow.com/ | 0 | IND |
+----+---------------+---------------------------+-------+---------+
mysql> SELECT * FROM access_log;
+-----+---------+-------+------------+---------+
| aid | site_id | count | date | country |
+-----+---------+-------+------------+---------+
| 1 | 1 | 45 | 2016-05-10 | USA |
| 2 | 3 | 100 | 2016-05-13 | JP |
| 3 | 1 | 230 | 2016-05-14 | CN |
| 4 | 2 | 10 | 2016-05-14 | USA |
| 5 | 5 | 205 | 2016-05-14 | CN |
| 6 | 4 | 13 | 2016-05-15 | USA |
| 7 | 3 | 220 | 2016-05-15 | USA |
| 8 | 5 | 545 | 2016-05-16 | USA |
| 9 | 3 | 201 | 2016-05-17 | IND |
| 10 | 6 | 100 | 2016-05-18 | ENG |
+-----+---------+-------+------------+---------+
"Websites" 表中的 "id" 列指向 "access_log" 表中的字段 "site_id"。上面这两个表是通过 "site_id" 列联系起来的。
INNER JOIN 关键字在表中存在至少一个匹配时返回行,可以看做是集合中的交集。
注释
INNER JOIN 与 JOIN 是相同的。
基础语法:
SELECT column_name(s)
FROM table1
INNER JOIN table2
ON table1.column_name=table2.column_name;
-- or
SELECT column_name(s)
FROM table1
JOIN table2
ON table1.column_name=table2.column_name;
比如:
SELECT Websites.name, access_log.count, access_log.date
FROM Websites
INNER JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count;
查询出的数据如下:
+--------------+--------+--------------+
| name | count | date |
+--------------+--------+--------------+
| 淘宝 | 10 | 2016-05-14 |
| 微博 | 13 | 2016-05-15 |
| Google | 45 | 2016-05-10 |
| 菜鸟教程 | 100 | 2016-05-13 |
| 菜鸟教程 | 201 | 2016-05-17 |
| Facebook | 205 | 2016-05-14 |
| 菜鸟教程 | 220 | 2016-05-15 |
| Google | 230 | 2016-05-14 |
| stackoverflow | 545 | 2016-05-16 |
+---------------+--------+--------------+
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。
注释
在某些数据库中,LEFT JOIN 称为 LEFT OUTER JOIN。
基础语法:
SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name=table2.column_name;
-- or
SELECT column_name(s)
FROM table1
LEFT OUTER JOIN table2
ON table1.column_name=table2.column_name;
比如:
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;
查询出的数据如下:
+--------------+--------+--------------+
| name | count | date |
+--------------+--------+--------------+
| 淘宝 | 10 | 2016-05-14 |
| 微博 | 13 | 2016-05-15 |
| Google | 45 | 2016-05-10 |
| 菜鸟教程 | 100 | 2016-05-13 |
| 菜鸟教程 | 201 | 2016-05-17 |
| Facebook | 205 | 2016-05-14 |
| 菜鸟教程 | 220 | 2016-05-15 |
| Google | 230 | 2016-05-14 |
| Facebook | 545 | 2016-05-16 |
| stackoverflow | NULL | NULL |
+---------------+--------+--------------+
RIGHT JOIN 关键字从右表(table2)返回所有的行,即使左表(table1)中没有匹配。如果左表中没有匹配,则结果为 NULL。
注释
在某些数据库中,RIGHT JOIN 称为 RIGHT OUTER JOIN。
基础语法:
SELECT column_name(s)
FROM table1
RIGHT JOIN table2
ON table1.column_name=table2.column_name;
-- or
SELECT column_name(s)
FROM table1
RIGHT OUTER JOIN table2
ON table1.column_name=table2.column_name;
比如:
SELECT Websites.name, access_log.count, access_log.date
FROM Websites
RIGHT JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count;
查询出的数据如下:
+--------------+--------+--------------+
| name | count | date |
+--------------+--------+--------------+
| 淘宝 | 10 | 2016-05-14 |
| 微博 | 13 | 2016-05-15 |
| Google | 45 | 2016-05-10 |
| 菜鸟教程 | 100 | 2016-05-13 |
| NULL | 100 | 2016-05-18 |
| 菜鸟教程 | 201 | 2016-05-17 |
| Facebook | 205 | 2016-05-14 |
| 菜鸟教程 | 220 | 2016-05-15 |
| Google | 230 | 2016-05-14 |
| Facebook | 545 | 2016-05-16 |
+---------------+--------+--------------+
FULL OUTER JOIN 关键字只要左表(table1)和右表(table2)其中一个表中存在匹配,则返回行.
FULL OUTER JOIN 关键字结合了 LEFT JOIN 和 RIGHT JOIN 的结果,相当于集合中的并集。
注释
在某些数据库中,RIGHT JOIN 称为 RIGHT OUTER JOIN。
基础语法:
SELECT column_name(s)
FROM table1
FULL OUTER JOIN table2
ON table1.column_name=table2.column_name;
比如:
SELECT Websites.name, access_log.count, access_log.date
FROM Websites
FULL OUTER JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count DESC;
查询出的数据如下:
+--------------+--------+--------------+
| name | count | date |
+--------------+--------+--------------+
| 淘宝 | 10 | 2016-05-14 |
| 微博 | 13 | 2016-05-15 |
| Google | 45 | 2016-05-10 |
| 菜鸟教程 | 100 | 2016-05-13 |
| NULL | 100 | 2016-05-18 |
| 菜鸟教程 | 201 | 2016-05-17 |
| Facebook | 205 | 2016-05-14 |
| 菜鸟教程 | 220 | 2016-05-15 |
| Google | 230 | 2016-05-14 |
| Facebook | 545 | 2016-05-16 |
| stackoverflow | NULL | NULL |
+---------------+--------+--------------+