如何在PostgreSQL中使用Case语句？

Question

那么，基本上我需要达到这个效果，有什么建议吗？

JOIN ((CASE WHEN (SELECT COUNT(e.order_id) FROM entry ee WHERE ee.order_id = o.id) > 1 
      THEN (SELECT * FROM entry e WHERE e.user_id = 3) 
      ELSE (SELECT * FROM entry e WHERE e.user_id = 3 AND e.draw_id = (SELECT dd.id 
                 FROM draw dd 
                 WHERE dd.draw_date_time > CURRENT_TIMESTAMP(0) 
                 ORDER BY dd.draw_date_time ASC 
                 LIMIT 1)) 
      END) e ON e.order_id = o.id 

Answer 1

不是你怎么做到的。把条件放在on条款中。如果我按照正确的逻辑：

JOIN 
(SELECT e.*, ROW_NUMBER() OVER (PARTITION BY ee.order_id) as cnt 
FROM entry e 
) e 
ON e.order_id = o.id and 
    e.user_id = 3 and 
    (e.cnt <= 1 or 
    e.draw_id = (SELECT dd.id 
       FROM draw dd 
       WHERE dd.draw_date_time > CURRENT_TIMESTAMP(0) 
       ORDER BY dd.draw_date_time ASC 
       LIMIT 1 
       ) 

我怀疑这可以进一步简化。如果没有更大的查询和逻辑应该做什么的解释，很难提出建议。你可能想问另一个问题。