应用左连接前过滤表 [英] Filter Table Before Applying Left Join

问题描述

我有 2 个表，我想在将 2 个表连接在一起之前过滤 1 个表.

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

客户表:

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

输入表:

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

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.但是我想在加入之前在条目表中过滤类别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.

预期结果:

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

如果我要进行以下查询:

If I was to do the following query:

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

这将过滤掉最后一条记录.

This would filter out the last record.

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.

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

提前感谢您的帮助！！

推荐答案

您需要将 WHERE 过滤器移动到 JOIN 条件:

You need to move the WHERE filter to the JOIN condition:

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

参见SQL Fiddle with Demo

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