SQL 应用左连接前过滤表

Filter Table Before Applying Left Join

提问by Tom Jenkin

I have 2 tables, I want to filter the 1 table beforethe 2 tables are joined together.

我有 2 个表，我想在 2 个表连接在一起之前过滤 1 个表。

Customer Table:

客户表：

   ╔══════════╦═══════╗
   ║ Customer ║ State ║
   ╠══════════╬═══════╣
   ║ A        ║ S     ║
   ║ B        ║ V     ║
   ║ C        ║ L     ║
   ╚══════════╩═══════╝

Entry Table:

条目表：

   ╔══════════╦═══════╦══════════╗
   ║ Customer ║ Entry ║ Category ║
   ╠══════════╬═══════╬══════════╣
   ║ A        ║  5575 ║ D        ║
   ║ A        ║  6532 ║ C        ║
   ║ A        ║  3215 ║ D        ║
   ║ A        ║  5645 ║ M        ║
   ║ B        ║  3331 ║ A        ║
   ║ B        ║  4445 ║ D        ║
   ╚══════════╩═══════╩══════════╝

I want to Left Join so I get all records from the Customer table regardless of whether there are related records in the Entry table. However I want to filter on category D in the entry table before the join.

我想Left Join，所以我从Customer 表中获取所有记录，而不管Entry 表中是否有相关记录。但是我想在 join 之前过滤条目表中的类别 D 。

Desired Results:

预期结果：

   ╔══════════╦═══════╦═══════╗
   ║ Customer ║ State ║ Entry ║
   ╠══════════╬═══════╬═══════╣
   ║ A        ║ S     ║  5575 ║
   ║ A        ║ S     ║  3215 ║
   ║ B        ║ V     ║  4445 ║
   ║ C        ║ L     ║  NULL ║
   ╚══════════╩═══════╩═══════╝

If I was to do the following query:

如果我要执行以下查询：

   SELECT Customer.Customer, Customer.State, Entry.Entry
   FROM Customer
   LEFT JOIN Entry
   ON Customer.Customer=Entry.Customer
   WHERE Entry.Category='D'

This would filter out the last record.

这将过滤掉最后一条记录。

So I want all rows from the left table and join it to the entry table filtered on category D.

所以我想要左表中的所有行并将其连接到按类别 D 过滤的条目表中。

Thanks to any help in advance!!

提前感谢任何帮助！！

回答by Taryn

You need to move the WHEREfilter to the JOINcondition:

您需要将WHERE过滤器移动到JOIN条件：

SELECT c.Customer, c.State, e.Entry
FROM Customer c
LEFT JOIN Entry e
   ON c.Customer=e.Customer
   AND e.Category='D'

See SQL Fiddle with Demo

参见SQL Fiddle with Demo

回答by Jeff Rosenberg

You could also do:

你也可以这样做：

SELECT c.Customer, c.State, e.Entry
FROM Customer AS c
LEFT JOIN (SELECT * FROM Entry WHERE Category='D') AS e
ON c.Customer=e.Customer

SQL Fiddle here

SQL小提琴在这里

回答by c z

Or...

或者...

SELECT c.Customer, c.State, e.Entry
FROM Customer c
LEFT JOIN Entry e
   ON c.Customer=e.Customer
WHERE e.Category IS NULL or e.Category='D'