朋友sql的精選朋友的性能（但不是我的朋友）

Question

我想得到不是我朋友的朋友的朋友。 我有一個這樣的表：userFriends（idUser，idUserFriend）

我在想這樣的事情：

select distinct idUserFriend from userFriends where idUser in
(select idUserFriend from userFriends where idUser = 1)
and idUserFriend not in (select idUserFriend from userFriends where idUser = 1)

但是我不確定它是否如此緩慢或是否可以以其他方式更快。

有什么看法嗎？

Answer 1

SELECT DISTINCT
    two.idUserFriend
FROM userFriends one
JOIN userFriends two ON one.idUserFriend = two.idUser
WHERE NOT EXISTS (
        SELECT * 
        FROM userFriends nx
        WHERE nx.idUser = one.idUser
        AND nx.idUserFriend = two.idUserFriend
        )
AND one.idUser = 1
        ;

可以通過“ NOT IN”構造實現相同的目的：

SELECT DISTINCT
    two.idUserFriend
FROM userFriends one
JOIN userFriends two ON one.idUserFriend = two.idUser
WHERE two.idUserFriend NOT IN (
        SELECT  nx.idUserFriend
        FROM userFriends nx
        WHERE nx.idUser = one.idUser
        )
AND one.idUser = 1
        ;

還有“ EXCEPT”版本：

SELECT DISTINCT
    two.idUserFriend
FROM userFriends one
JOIN userFriends two ON one.idUserFriend = two.idUser
WHERE one.idUser = 1
EXCEPT (
        SELECT  nx.idUserFriend
        FROM userFriends nx
        WHERE nx.idUser = 1
        )
        ;

Answer 2

使用聯接進行比較：

SELECT
    b.idUserFriend
FROM 
    userFriends a
INNER JOIN
    userFriends b ON a.idUserFriend = b.idUser
LEFT JOIN
    userFriends c ON a.idUser = c.idUser AND b.idUserFriend = c.idUserFriend
WHERE
    a.idUser = 1
    AND c.idUser IS NULL
GROUP BY
    b.idUserFriend #eliminate duplicates

通常，最好在子查詢上使用JOIN ，因為它們將利用相關字段上的索引進行比較。 另一方面，子查詢將為返回的每一行執行，即使它是不相關的子查詢（MySQL）。

有關JOIN與子查詢的更多信息

Answer 3

select myFriend.idUserFriend from userFriend me inner join userFriends myFriend 
on me.idUserFriend = myFriend.idUser and myFriend.idUserFriend != me.idUserFriend 
where me.idUser='MyUserId';

Answer 4

嘗試這個 -

SELECT UF2.idUser, UF2.idUserFriend 
FROM USERFIRENDS UF2 INNER JOIN 
(
    SELECT idUser, idUserFriend
    FROM USERFRIENDS UF1
    WHERE UF1.idUser =1 -- this query gives my friend
) MYFRIENDS ON 
      UF2.idUser=MYFRIENDS.idUserFriend -- get my freinds friends
      AND UF2.idUserFriend NOT IN 
           (
              SELECT idUserFriend
              FROM USERFRIENDS UF1
              WHERE UF1.idUser =1           
           )

該查詢的優點在於，第一個子查詢是聯接的一部分，並且不會為每個記錄執行。

不幸的是，您將需要為NOT IN情況使用子查詢。