加入MySQL中的CASE

Question

試圖在MySQL中實現多態關聯。我認爲代碼可能是這個樣子：

SELECT 
... 
FROM 
resource 
LEFT JOIN (
    CASE resource.owner_type 
     WHEN 'meeting' THEN 
     (SELECT * FROM meeting) as owner 
     WHEN 'todo' THEN 
     (SELECT * FROM todo) as owner 
     END) 

但此案引起語法錯誤。這怎麼能被正確地重寫？

Answer 1

簡短的回答是，你不能代替連接在行通過行的基礎上 - 但我相信這是你想達到什麼樣的解決方案：

SELECT 
... 
FROM 
resource 
LEFT JOIN meeting ON resource.id=meeting.resource_id 
WHERE resource.owner_type='meeting' 
UNION 
SELECT 
... 
FROM 
resource 
LEFT JOIN todo ON resource.id=todo.resource_id 
WHERE resource.owner_type='todo'