MySQL Essentials: Creating Advanced Joins

1. Use table aliases

In addition to aliases for column names and calculated fields, SQL also allows aliases for table names. There are two main reasons for doing this:

• Shorten the SQL statement;

• Allow the same table to be used multiple times in a single SELECT statement.

mysql> SELECT cust_name, cust_contact 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='TNT2';
+----------------+--------------+
| cust_name      | cust_contact |
+----------------+--------------+
| Coyote Inc.    | Y Lee        |
| Yosemite Place | Y Sam        |
+----------------+--------------+

• Analysis: You can see that all three tables in the FROM clause have aliases. customers AS c Create c as an alias for customers, and so on. This enables the use of the abbreviated c instead of the full name customers. In this example, the table alias is used only in the WHERE clause. However, table aliases can be used not only in the WHERE clause, but also in lists of SELECT, ORDER BY clauses, and other parts of the statement.

2. Use different types of joins

2.1 Self-join

mysql> 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'
    -> ;
+---------+----------------+
| prod_id | prod_name      |
+---------+----------------+
| DTNTR   | Detonator      |
| FB      | Bird seed      |
| FC      | Carrots        |
| SAFE    | Safe           |
| SLING   | Sling          |
| TNT1    | TNT (1 stick)  |
| TNT2    | TNT (5 sticks) |
+---------+----------------+

• Analysis: The two required in this query The tables are actually the same table, so the products table appears twice in the FROM clause. Although this is perfectly legal, the reference to products is ambiguous because MySQL does not know which instance in the products table you are referencing.

Use self-joins instead of subqueries: Self-joins are often used as external statements to replace subquery statements used when retrieving data from the same table. Although the final result is the same, sometimes processing joins is much faster than processing subqueries

2.2 Natural joins

Natural joins exclude multiple occurrences so that each column Return only once.

mysql> 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';
+---------+-------------+----------------+-----------+------------+----------+--------------+--------------+-----------------+-----------+---------------------+---------+----------+------------+
| cust_id | cust_name   | cust_address   | cust_city | cust_state | cust_zip | cust_country | cust_contact | cust_email      | order_num | order_date          | prod_id | quantity | item_price |
+---------+-------------+----------------+-----------+------------+----------+--------------+--------------+-----------------+-----------+---------------------+---------+----------+------------+
|   10001 | Coyote Inc. | 200 Maple Lane | Detroit   | MI         | 44444    | USA          | Y Lee        | ylee@coyote.com |     20005 | 2005-09-01 00:00:00 | FB      |        1 |      10.00 |
|   10001 | Coyote Inc. | 200 Maple Lane | Detroit   | MI         | 44444    | USA          | Y Lee        | ylee@coyote.com |     20009 | 2005-10-08 00:00:00 | FB      |        1 |      10.00 |
+---------+-------------+----------------+-----------+------------+----------+--------------+--------------+-----------------+-----------+---------------------+---------+----------+------------

• Analysis: In this example, wildcards are only used for the first table. All other columns are listed explicitly so no duplicate columns are retrieved.

2.3 External connection

许多联结将一个表中的行与另一个表中的行相关联。但有时候会需要包含没有关联行的那些行。
为了检索所有客户，包括那些没有订单的客户，可如下进行：

mysql> SELECT customers.cust_id, orders.order_num FROM customers LEFT OUTER JOIN orders ON omers.cust_id = orders.cust_id;
+---------+-----------+
| cust_id | order_num |
+---------+-----------+
|   10001 |     20005 |
|   10001 |     20009 |
|   10002 |      NULL |
|   10003 |     20006 |
|   10004 |     20007 |
|   10005 |     20008 |
+---------+-----------+

• 分析：类似于上一章中所看到的内部联结，这条SELECT语句使用了关键字OUTER JOIN来指定联结的类型（而不是在WHERE子句中指定）。但是，与内部联结关联两个表中的行不同的是，外部联结还包括没有关联行的行。在使用OUTER JOIN语法时，必须使用RIGHT或LEFT关键字指定包括其所有行的表（ RIGHT指出的是OUTER JOIN右边的表，而LEFT指出的是OUTER JOIN左边的表）。 上面的例子使用LEFT OUTER JOIN从FROM子句的左边表（customers表）中选择所有行。

3 使用带聚集函数的联结

如果要检索所有客户及每个客户所下的订单数，下面使用了COUNT()函数的代码可完成此工作：

mysql> SELECT customers.cust_name, customers.cust_id, COUNT(orders.order_num) AS num_ord FROM customers INNER JOIN orders ON customers.cust_id = orders.cust_id GROUP BY customers.cust_id;
+----------------+---------+---------+
| cust_name      | cust_id | num_ord |
+----------------+---------+---------+
| Coyote Inc.    |   10001 |       2 |
| Wascals        |   10003 |       1 |
| Yosemite Place |   10004 |       1 |
| E Fudd         |   10005 |       1 |
+----------------+---------+---------+

• 分析：此SELECT语句使用INNER JOIN将customers和orders表互相关联。GROUP BY 子 句 按 客 户 分 组 数 据 ， 因 此 ， 函 数 调 用COUNT(orders.order_num)对每个客户的订单计数，将它作为num_ord返回。

mysql> SELECT customers.cust_name, customers.cust_id, COUNT(orders.order_num) AS num_ord FROM customers LEFT OUTER JOIN orders ON customers.cust_id = orders.cust_id GROUP BY customers.cust_id;
+----------------+---------+---------+
| cust_name      | cust_id | num_ord |
+----------------+---------+---------+
| Coyote Inc.    |   10001 |       2 |
| Mouse House    |   10002 |       0 |
| Wascals        |   10003 |       1 |
| Yosemite Place |   10004 |       1 |
| E Fudd         |   10005 |       1 |
+----------------+---------+---------+

• 分析：这个例子使用左外部联结来包含所有客户，甚至包含那些没有任何下订单的客户。结果显示也包含了客户Mouse House，它有0个订单。

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