Question

我有一张包含一些数据的表

t1
------------------
id (int)
item_id (int)
item_description (text)
showHide (enum 0,1)

通常我会显示showHide标志设置为'0'的任何结果。如果用户在另一个表t2中有相应的item_id，我需要修改查询以显示showHide标志设置为'1'的结果。

t2
------------------
id (int)
item_id (int)
usr_id (int)

坚持到这里，不知道如何做到这一点：

SELECT item_description FROM t1
// get results with  showHide = 0 
// AND 
// showHide = 1 if t1.item_id found in t2 WHERE usr_id = 123

Answer 1

只需将2个查询与UNION

组合在一起

SELECT t1.item_description
  FROM t1
 WHERE showHide = 0
UNION
    SELECT t1.item_description
      FROM t1
INNER JOIN t2 ON t1.item_id = t2.item_id
     WHERE showHide = 1

更棘手的方式，因此性能较差：

    SELECT t1.item_description
      FROM t1
 LEFT JOIN t2 ON t1.item_id = t2.item_id
     WHERE showHide = 0
        OR (showHide = 1 AND t2.item_id IS NOT NULL)

Answer 2

SELECT 
  t1.item_description
FROM
  t1
LEFT JOIN (
  SELECT item_id FROM t2 WHERE usr_id = 123
) AS t2
USING (item_id)
WHERE 
  t1.showHide = 0 
OR
  (t1.showHide = 1 AND t2.item_id IS NOT NULL)

mySQL查询 - 条件选择

2 个答案: