15.4 What are the different types of joins? Please explain how they differ and why certain types are better in certain situations.
Join是用来联合两个表的，每个表至少需要有一列是相同的，不同的Join类型会返回不同的结果。我们来看一个例子，有两个表，普通饮料和无卡饮料如下：
-- TABLE RegularBeverages
+-----------+-----------+
| Name | Code |
+-----------+-----------+
| Budweiser | BUDWEISER |
| Coca-Cola | COCACOLA |
| Pepsi | PEPSI |
+-----------+-----------+
-- TABLE CalorieFreeBeverages
+----------------+----------+
| Name | Code |
+----------------+----------+
| Diet Coca-Cola | COCACOLA |
| Fresca | FRESCA |
| Diet Pepsi | PEPSI |
| Pepsi Light | PEPSI |
| Purfied water | Water |
+----------------+----------+
如果我们想联合这两个表，有很多种Join可以使用：
内交Inner Join：只会显示相同列匹配的项：
SELECT * FROM RegularBeverages
INNER JOIN CalorieFreeBeverages
ON RegularBeverages.Code = CalorieFreeBeverages.Code; +-----------+----------+----------------+----------+
| Name | Code | Name | Code |
+-----------+----------+----------------+----------+
| Coca-Cola | COCACOLA | Diet Coca-Cola | COCACOLA |
| Pepsi | PEPSI | Diet Pepsi | PEPSI |
| Pepsi | PEPSI | Pepsi Light | PEPSI |
+-----------+----------+----------------+----------+
外交Outer Join：外交会包含内交的结果，同时也会包含一些没有匹配到的结果，外交有如下几种：
- 左交Left Outer Join (Left Join)，会包含左表的所有结果，如果没有匹配，右边就是NULL：
SELECT * FROM RegularBeverages
LEFT JOIN CalorieFreeBeverages
ON RegularBeverages.Code = CalorieFreeBeverages.Code; +-----------+-----------+----------------+----------+
| Name | Code | Name | Code |
+-----------+-----------+----------------+----------+
| Coca-Cola | COCACOLA | Diet Coca-Cola | COCACOLA |
| Pepsi | PEPSI | Diet Pepsi | PEPSI |
| Pepsi | PEPSI | Pepsi Light | PEPSI |
| Budweiser | BUDWEISER | NULL | NULL |
+-----------+-----------+----------------+----------+
- 右交Right Outer Join (Right Join)，返回右表的所有项，如果没有匹配，左边就是NULL：
SELECT * FROM RegularBeverages
RIGHT JOIN CalorieFreeBeverages
ON RegularBeverages.Code = CalorieFreeBeverages.Code; +-----------+----------+----------------+----------+
| Name | Code | Name | Code |
+-----------+----------+----------------+----------+
| Coca-Cola | COCACOLA | Diet Coca-Cola | COCACOLA |
| Pepsi | PEPSI | Diet Pepsi | PEPSI |
| Pepsi | PEPSI | Pepsi Light | PEPSI |
| NULL | NULL | Fresca | FRESCA |
| NULL | NULL | Purfied water | Water |
+-----------+----------+----------------+----------+
- 全交Full Outer Join，联合左交和右交的结果，不论有没有匹配都把结果显示出来，由于MySQL中没有这个命令，所以我们用Union把左右交的结果并起来：
SELECT * FROM RegularBeverages
LEFT JOIN CalorieFreeBeverages
ON RegularBeverages.Code = CalorieFreeBeverages.Code
UNION
SELECT * FROM RegularBeverages
RIGHT JOIN CalorieFreeBeverages
ON RegularBeverages.Code = CalorieFreeBeverages.Code; +-----------+-----------+----------------+----------+
| Name | Code | Name | Code |
+-----------+-----------+----------------+----------+
| Coca-Cola | COCACOLA | Diet Coca-Cola | COCACOLA |
| Pepsi | PEPSI | Diet Pepsi | PEPSI |
| Pepsi | PEPSI | Pepsi Light | PEPSI |
| Budweiser | BUDWEISER | NULL | NULL |
| NULL | NULL | Fresca | FRESCA |
| NULL | NULL | Purfied water | Water |
+-----------+-----------+----------------+----------+
关于此内容还可以参见我之前的一篇博客SQL Left Join, Right Join, Inner Join, and Natural Join 各种Join小结。