是否存在MySQL查询

Question

我正在尝试执行一个简单的IF，否则IF语句，这在SQL中相当容易。出于某种原因，它不会在MySQL上听我的话。它一直告诉我“看他妈的手册”还有手册：https://dev.mysql.com/doc/refman/5.7/en/exists-and-not-exists-subqueries.html

不说明如何在SELECT部分中这样做，只在where之后，这意味着我不能以我习惯的方式来做这件事。剩下的问题是，那我该怎么做呢？

    SELECT EXISTS(SELECT 1 FROM `content` WHERE `ResourceKey`='Test' AND `Language`='en-GB') 
THEN
    SELECT `Text` FROM `content` WHERE `ResourceKey`='Test' AND `Language`='en-GB'; 
ELSE 
    SELECT `Text` FROM `content` WHERE `ResourceKey`='Test' AND `Language`='nl-NL';
END IF;

还有一些信息：

Innodb version: 1.1.8 Protocol version: 10

Answer 1

如果您希望从查询中获得一条记录，则不需要为此使用IF ELSE语句。试一试：

SELECT `Text` 
FROM `content` 
WHERE `ResourceKey`='Test' AND `Language` ΙΝ ('en-GB', 'nl-NL');
ORDER BY CASE 
            WHEN `Language` = 'en-GB' THEN 1 
            ELSE 2 
         END
LIMIT 1

如果您希望始终返回某些文本，即使没有返回记录，则请使用：

SELECT COALESCE(t2.Text, 'No record found') AS Text
FROM (
   SELECT 'en-GB' AS lang UNION ALL 'nl-NL' UNION ALL 'no-lang') AS t1
LEFT JOIN content AS t2 
   ON t1.lang = t2.Language AND t2.ResourceKey = 'Test'  
ORDER BY CASE 
            WHEN t1.lang = 'en-GB' THEN 1 
            WHEN t1.lang = 'nl-NL' THEN 2
            ELSE 3         -- This is the case where t1.lang = 'no-lang'
         END
LIMIT 1

我假设您的数据库中没有Language = 'no-lang'的记录。

Answer 2

SELECT `Text` 
FROM `content` 
WHERE `ResourceKey` = 'Test'
AND `Language` ΙΝ ('en-GB', 'nl-NL')
ORDER BY if (`Language` = 'en-GB', 1, 2)
LIMIT 1