JPQL：如果更新不存在

Question

我想實現這樣的五月JPA /休眠/ MySQL的應用：

UPDATE UserRating ur SET ur.item = :newItem WHERE ur.item = :oldItem AND NOT EXISTS (SELECT ur2 FROM UserRating ur2 WHERE ur.user = ur2.user AND ur2.item = :newItem) 

所以，我想更新一些行，只有當他們將是唯一的（用戶/項目組合必須是唯一的）。

我越來越

Caused by: java.sql.SQLException: You can't specify target table 'user_rating' for update in FROM clause 

當前JPQL。

Answer 1

一種方法是把它變成JOIN

UPDATE UserRating r JOIN 
( 
    SELECT user, item 
    FROM UserRating ur 
    WHERE item = :oldItem 
    AND NOT EXISTS 
    (
    SELECT * 
    FROM UserRating 
    WHERE user = ur.user 
     AND item = :newItem 
) 
) q ON r.user = q.user 
    AND r.item = q.item 
    SET r.item = :newItem 

下面是SQLFiddle演示