Question

我有3张桌子：

一个拥有“位置”信息。
其他包含有关用户首选的“位置”（来自位置）的信息
其他包含有关用户访问过的位置的信息。

我想显示访问的地址信息，但是如果不可用，请使用用户首选位置的地址信息。

我在为左联接设置条件时遇到麻烦（如果一个条件不可用，则从另一张桌子中抢走）

这是我的查询：

SELECT rew.cust_id,
    rew.location_name,
    CASE WHEN rew.used_location IS NULL THEN customer.home_location ELSE rew.used_location END, 
    locations.street_address,
    locations.city,
    locations.zip_code FROM rew
INNER JOIN customer ON rew.cust_id = customer.cust_id
LEFT JOIN locations ON rew.used_location = locations.location_name
WHERE rew.cust_id = 12

此打印以下内容

cust_id    |   location_name   |   used_location    |   street_address   | city   |   zip_code
-----------------------------------------------------------------------------------------------
   12      |   place 1         |     place 1        |   123 My Street    | City   |     00000
   12      |   place 1         |     place 1        |   NULL             | NULL   |     NULL
   12      |   place 1         |     place 1        |   NULL             | NULL   |     NULL

在此查询中，第二条记录和第三条记录的used_location实际上是customer.home_location的条件，但它没有连接地址上的信息。如何设置为null，然后将customer.home_location中的地址放在JOIN上的位置

Answer 1

尝试一下：

SELECT rew.cust_id,
    rew.location_name,
    CASE WHEN rew.used_location IS NULL THEN customer.home_location ELSE rew.used_location END, 
    locations.street_address,
    locations.city,
    locations.zip_code FROM rew
INNER JOIN customer ON rew.cust_id = customer.cust_id
LEFT JOIN locations ON 
    rew.used_location = locations.location_name
    AND rew.used_location IS NOT NULL
    OR customer.home_location = locations.location_name
    AND rew.used_location IS NULL
WHERE rew.cust_id = 12

Answer 2

我认为您想在customer和rew之间进行LEFT JOIN（在这个方向上，对于每个客户来说，都有可能访问过该位置-或者在rew和{ {1}}），然后将其与customer表结合起来以获取有关所选位置的信息。

查询如下：

locations

或者，嵌套查询更加清晰：

SELECT customer.cust_id,
    locations.location_name,
    locations.street_address,
    locations.city,
    locations.zip_code
FROM customer
LEFT JOIN rew USING (cust_id)
INNER JOIN locations ON COALESCE(rew.used_location, customer.home_location) = locations.location_name
WHERE customer.cust_id = 12

我也将SELECT result.cust_id, locations.location_name, locations.street_address, locations.city, locations.zip_code FROM ( SELECT customer.cust_id, COALESCE(rew.used_location, customer.home_location) AS location_name FROM customer LEFT JOIN rew USING (cust_id) WHERE customer.cust_id = 12 ) AS result INNER JOIN locations USING (location_name)表达式简化为CASE调用。

Answer 3

您的地址连接始终在rew.used_location列上（根据您的定义，该列可能为NULL）。您需要使连接成为条件，类似于使第三显示列成为条件。

SELECT rew.cust_id,
       CASE WHEN rew.used_location IS NULL THEN customer.home_location ELSE rew.used_location END AS location_to_show,
       rew.location_name,
       locations.street_address,
       locations.city,
       locations.zip_code FROM rew
INNER JOIN customer ON rew.cust_id = customer.cust_id
LEFT JOIN locations ON  
  CASE 
    WHEN rew.used_location IS NULL THEN customer.home_location
    ELSE rew.used_location
    END = locations.location_name

SQL Fiddle To Try This

以LEFT JOIN PostgreSQL为条件

3 个答案: