SQL内部联接和计数

Question

我有3个表（用户，项目，类似用户）和2个sql查询。 如何统一这两个查询？

SELECT item.userid, item.id, user.name FROM item 
INNER JOIN user ON item.userid = user.id 

SELECT userid,itemid, COUNT(*) AS `liked` FROM userlike
WHERE userid=9
GROUP BY itemid

我想知道特定用户（9）是否喜欢该商品。

结果应该是这样的

itemid userid name liked* (*whether 'user 9' liked this item or not)
1      7      foo  0
2      4      asd  1

谢谢

Answer 1

您要为此使用OUTER JOIN

SELECT i.id itemid, u.id userid, u.name, COALESCE(liked, 0) liked
  FROM item i JOIN user u
    ON i.userid = u.id LEFT  JOIN 
(
  SELECT itemid, COUNT(*) liked 
    FROM userlike 
   WHERE userid = 9
   GROUP BY itemid
) l
    ON i.id = l.itemid;

要么

SELECT i.id itemid, u.id userid, u.name, l.userid IS NOT NULL liked
  FROM item i JOIN user u
    ON i.userid = u.id LEFT  JOIN userlike l
    ON i.id = l.itemid
   AND l.userid = 9;

样本输出：

| ITEMID | USERID |  NAME | LIKED |
|--------|--------|-------|-------|
|      2 |      4 | user4 |     1 |
|      1 |      7 | user7 |     0 |

这是SQLFiddle演示

Answer 2

SELECT item.id, item.userid, user.name, userlike.liked
FROM item
JOIN user ON user.id = item.userid
JOIN userlike ON item.id = userlike.itemid
WHERE userlike.liked = 1
GROUP BY item.id

要么

SELECT item.id, item.userid, user.name, userlike.liked
FROM item
JOIN user ON user.id = item.userid
JOIN userlike ON item.id = userlike.itemid
WHERE COUNT(userlike.liked) >= 1
GROUP BY item.id

Answer 3

您不需要使用COUNT 。 您只需要知道在userlike表中是否存在特定项目和特定用户的userlike ：

SELECT i.id as itemid, u.id as userid, u.name,
       case when ul.userid is null then 0 else 1 end as liked
FROM item i
INNER JOIN user u ON i.userid = u.id
LEFT OUTER JOIN userlike ul ON i.id = ul.itemid AND ul.userid=9
ORDER BY i.id