MYSQL 案例

Question

我进退两难，以下 SQL 查询的结果是什么以及它是如何工作的:

SELECT ...
CASE 
     WHEN (a.FIELD=1 AND b.FIELD=2) THEN 1
     WHEN (a.FIELD=1 AND b.FIELD=3) THEN 3
     WHEN (b.FIELD=2 AND c.FIELD=1) THEN 0
        END) 

..

FROM a
LEFT JOIN b ON a.ID = b.FOREIGN_KEY
LEFT JOIN c ON b.ID = c.FOREIGN_KEY

WHERE <conditions>

所以基本上有更多条件适用于 CASE 语句。MySQL 如何评估它们？结果是第一个 TRUE 条件还是最后一个？

谢谢

PS:我正在查看 MySQL 文档，但没有找到相关提示。

Answer 1

第一个匹配条件获胜。

CASE

CASE
    WHEN search_condition THEN statement_list
    [WHEN search_condition THEN statement_list] ...
    [ELSE statement_list]
END CASE

each WHEN clause search_condition expression is evaluated until one is true, at which point its corresponding THEN clause statement_list executes. If no search_condition is equal, the ELSE clause statement_list executes, if there is one.