使用CASE和INNER JOIN的MySQL查询错误 [英] MySQL query error with CASE and INNER JOIN
问题描述
我有一个表,其中包含名为"a"的用户信息,以及一个表,其中包含来自不同API(twitter,foursquare)的数据,该示例中基于值 a.api_type的值应变为b.最后,我想要的是能够从活动的API(api_foursquare或api_twitter)中获取正确的头像.
I have a table with user info called 'a' and a table with data from different API's (twitter,foursquare) which in the example, based on the value of the a.api_type should become b. What I want in the end is to be able to grab the right avatar from the active API (either api_foursquare or api_twitter).
一段时间以来,我一直试图使它与该查询一起使用,但是我一直收到此错误. Sql不是我的强项,所以任何有关解决此问题的技巧都将很不错:)
I have been trying to get this to work with this query for a while, but I keep getting this error. Sql is not my strongest point, so any tips on how to fix this would be great :)
"[Err] 1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near
INNER JOIN b ON b.user_id = a.user_id at line 11"
SELECT a.user_id,
a.api_type,
b.avatar,
b.user_id
(CASE
WHEN a.api_type = 0 THEN api_foursquare
WHEN a.api_type = 1 THEN api_twitter
END) as b
FROM a WHERE a.cookie_hash = :cookie_hash
INNER JOIN b ON b.user_id = a.user_id
推荐答案
您不能选择要加入case语句的表.您应该左连接到两个表,然后在case
语句中从其中一个表中选择值,如下所示:
You cannot pick a table to join in a case statement. You should left-join to both tables, and then pick the value from one of them in the case
statement, like this:
SELECT a.user_id,
a.api_type,
(case WHEN a.api_type = 0 THEN b.avatar ELSE c.avatar END) as avatar,
(case WHEN a.api_type = 0 THEN b.user_id ELSE c.user_id END) as user_id
FROM a
LEFT OUTER JOIN api_foursquare b ON b.user_id = a.user_id
LEFT OUTER JOIN api_twitter c ON c.user_id = a.user_id
WHERE a.cookie_hash = :cookie_hash
您可能不需要最后一个表达式(...as user_id
一个),因为如果api_twitter
或api_foursquare
中存在与a.user_id
相匹配的行,则它将等于a.user_id
.
You probably do not need the last expression (the ...as user_id
one), because it is going to be equal to a.user_id
if there is a row in either api_twitter
or api_foursquare
that matches a.user_id
.
您还必须将WHERE
子句放在FROM
子句之后:
You also have to put the WHERE
clause after the FROM
clause:
编辑:考虑到ypercube的出色建议,查询看起来像这样:
Taking into account ypercube's great suggestion, the query would look like this:
SELECT a.user_id,
a.api_type,
COALESCE(b.avatar, c.avatar) as avatar
FROM a
LEFT OUTER JOIN api_foursquare b ON b.user_id = a.user_id AND a.api_type = 0
LEFT OUTER JOIN api_twitter c ON c.user_id = a.user_id and a.api_type = 1
WHERE a.cookie_hash = :cookie_hash
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