MySQL Crash Course #08# Chapter 16. Using Different Join Types

记文档还是相当重要的！

索引

假名的三个用途
自交（Self Joins）
自然交（Natural Joins）
Outer Joins

Using Table Aliases

Using aliases for column names and calculated fields
To shorten the SQL syntax
To enable multiple uses of the same table within a single SELECT statement

自交

像下面这样的叫做“自交” ↓

SELECT p1.prod_id, p1.prod_name
FROM products AS p1, products AS p2
WHERE p1.vend_id = p2.vend_id
  AND p2.prod_id = 'DTNTR';

Self Joins Instead of Subqueries Self joins are often used to replace statements using subqueries that retrieve data from the same table as the outer statement. Although the end result is the same, sometimes these joins execute far more quickly than they do subqueries. It is usually worth experimenting with both to determine which performs better.

接下来用于实验的两张表：

-- master 
+-----------+-------------+
| master_id | master_name |
+-----------+-------------+
|       | 王二牠       |
|       | 李明顠       |
|       | 田中吠       |
|       | 陆大襠       |
+-----------+-------------+

-- pet
+--------+----------+----------+-----------+
| pet_id | pet_type | pet_name | master_id |
+--------+----------+----------+-----------+
|    | NULL     | 飿¡¶        |       |
|    | dog      | 小白     |       |
|    | cat      | 老黄     |       |
+--------+----------+----------+-----------+

无约束自交，原来 3 条结果 3 × 3 ↓

mysql> SELECT *
    -> FROM pet AS p1, pet AS p2;
+--------+----------+----------+-----------+--------+----------+----------+-----------+
| pet_id | pet_type | pet_name | master_id | pet_id | pet_type | pet_name | master_id |
+--------+----------+----------+-----------+--------+----------+----------+-----------+
|   8881 | NULL     | 飿¡¶        |      1001 |   8881 | NULL     | 飿¡¶        |      1001 |
|   8882 | dog      | 小白     |      1002 |   8881 | NULL     | 飿¡¶        |      1001 |
|   8883 | cat      | 老黄     |      1003 |   8881 | NULL     | 飿¡¶        |      1001 |
|   8881 | NULL     | 飿¡¶        |      1001 |   8882 | dog      | 小白     |      1002 |
|   8882 | dog      | 小白     |      1002 |   8882 | dog      | 小白     |      1002 |
|   8883 | cat      | 老黄     |      1003 |   8882 | dog      | 小白     |      1002 |
|   8881 | NULL     | 飿¡¶        |      1001 |   8883 | cat      | 老黄     |      1003 |
|   8882 | dog      | 小白     |      1002 |   8883 | cat      | 老黄     |      1003 |
|   8883 | cat      | 老黄     |      1003 |   8883 | cat      | 老黄     |      1003 |
+--------+----------+----------+-----------+--------+----------+----------+-----------+

自然交。。

就是可以自动消除相同字段的一种交，但是 MySQL 并没有实现这种交，如果你像下面这样查就会看到好多相同字段：

SELECT *
FROM customers AS c, orders AS o, orderitems AS oi
WHERE c.cust_id = o.cust_id
  AND oi.order_num = o.order_num
  AND prod_id = 'FB';

所以，要实现自然交就只能自己具体指明字段了：

SELECT c.*, o.order_num, o.order_date,
       oi.prod_id, oi.quantity, OI.item_price
FROM customers AS c, orders AS o, orderitems AS oi
WHERE c.cust_id = o.cust_id
  AND oi.order_num = o.order_num
  AND prod_id = 'FB';

MySQL 不实现自然交：

mysql> SELECT *
    -> FROM master AS m, pet AS p
    -> WHERE m.master_id = p.master_id;
+-----------+-------------+--------+----------+----------+-----------+
| master_id | master_name | pet_id | pet_type | pet_name | master_id |
+-----------+-------------+--------+----------+----------+-----------+
|      1001 | 王二牠       |   8881 | NULL     | 飿¡¶        |      1001 |
|      1002 | 李明顠       |   8882 | dog      | 小白     |      1002 |
|      1003 | 田中吠       |   8883 | cat      | 老黄     |      1003 |
+-----------+-------------+--------+----------+----------+-----------+
3 rows in set (0.00 sec)

实现自然交：

mysql> SELECT master_name, p.*
    -> FROM master AS m, pet AS p
    -> WHERE m.master_id = p.master_id;
+-------------+--------+----------+----------+-----------+
| master_name | pet_id | pet_type | pet_name | master_id |
+-------------+--------+----------+----------+-----------+
| 王二牠       |   8881 | NULL     | 飿¡¶        |      1001 |
| 李明顠       |   8882 | dog      | 小白     |      1002 |
| 田中吠       |   8883 | cat      | 老黄     |      1003 |
+-------------+--------+----------+----------+-----------+

PS. FROM 里面 AS 的假名在 SELECT 中是可以用的。

Outer Joins

The join includes table rows that have no associated rows in the related table. This type of join is called an outer join. But unlike inner joins, which relate rows in both tables, outer joins also include rows with no related rows.

在讲到 OUTER JOIN 的同时就不得不提到 INNER JOIN ，推荐阅读这篇文章 Inner Join vs. Outer Join

真 · INNER JOIN ↓

mysql> SELECT master_name, p.*
    -> FROM master AS m INNER JOIN pet AS p
    -> ON m.master_id = p.master_id;
+-------------+--------+----------+----------+-----------+
| master_name | pet_id | pet_type | pet_name | master_id |
+-------------+--------+----------+----------+-----------+
| 王二牠       |   8881 | NULL     | 飿¡¶        |      1001 |
| 李明顠       |   8882 | dog      | 小白     |      1002 |
| 田中吠       |   8883 | cat      | 老黄     |      1003 |
+-------------+--------+----------+----------+-----------+

因为我个人认为没有人会需要 X * X 的那种表所以自己把那种不加 ON 后面的相等约束的表叫伪 · INNER JOIN

LEFT OUTER JOIN ↓

mysql> SELECT master_name, p.*
    -> FROM master AS m LEFT OUTER JOIN pet AS p
    -> ON m.master_id = p.master_id;
+-------------+--------+----------+----------+-----------+
| master_name | pet_id | pet_type | pet_name | master_id |
+-------------+--------+----------+----------+-----------+
| 王二牠       |   8881 | NULL     | 飿¡¶        |      1001 |
| 李明顠       |   8882 | dog      | 小白     |      1002 |
| 田中吠       |   8883 | cat      | 老黄     |      1003 |
| 陆大襠       |   NULL | NULL     | NULL     |      NULL |
+-------------+--------+----------+----------+-----------+

RIGHT OUTER JOIN ↓

mysql> SELECT master_name, p.*
    -> FROM master AS m RIGHT OUTER JOIN pet AS p
    -> ON m.master_id = p.master_id;
+-------------+--------+----------+----------+-----------+
| master_name | pet_id | pet_type | pet_name | master_id |
+-------------+--------+----------+----------+-----------+
| 王二牠       |   8881 | NULL     | 飿¡¶        |      1001 |
| 李明顠       |   8882 | dog      | 小白     |      1002 |
| 田中吠       |   8883 | cat      | 老黄     |      1003 |
+-------------+--------+----------+----------+-----------+

PS. the two types of outer join can be used interchangeably, and the decision about which one is used is based purely on convenience.