mysql - Mysql IF NOT EXISTS 然后选择

Question

我尝试按 parent_id 和语言从表中选择所有行“标签”。默认语言为 en。

list = SELECT * FROM labels WHERE parent_id = x AND lang='de'

if( list.isEmpty() ){
   list = SELECT * FROM labels WHERE parent_id = x AND lang='en'
}

如何只使用一个 MySQL 语句来处理这个问题？

非常感谢

Answer 1

使用where not exists和union all：

SELECT * FROM labels WHERE parent_id = x AND lang='de'
union all
SELECT *
FROM labels
WHERE parent_id = x AND lang = 'en' and
      not exists (SELECT * FROM labels WHERE parent_id = x AND lang='de')

Answer 2

试试这个，

SELECT  *
FROM    labels
WHERE   parent_id = x AND
        lang = 
        (
            SELECT  lang
            FROM    labels
            WHERE   parent_id = x
            ORDER   BY FIELD(lang, 'en', 'de') DESC
            LIMIT   1 
        )

• SQLFiddle 演示

FIELD()函数返回列表中字符串的索引。

这里使用JOIN

SELECT  DISTINCT a.*
FROM    labels a
        INNER JOIN
        (
            SELECT  parent_id, lang
            FROM    labels
            WHERE   parent_id = 1
            ORDER   BY FIELD(lang, 'en', 'de') DESC
            LIMIT   1 
        ) b ON a.parent_id = b.parent_id AND
                a.lang = b.lang
WHERE   a.parent_id = 1 

• SQLFiddle 演示