Question

请查看此查询，并尝试向我提供任何其他可以更有效地提供完全相同结果的提示。

SELECT username
FROM users
WHERE username NOT IN (
    SELECT DISTINCT username
    FROM users, friends 
    WHERE 'user1' IN (you,friend,username) 
      AND you     IN ('user1',username) 
      AND friend  IN ('user1',username))
  AND username <> 'user1'

Here's a live demo

Answer 1

当然效率不高 - 你的子选择中有笛卡尔。从您的查询中不是特别清楚您实际想要实现的目标。如果你描述了这个问题，它将会更有帮助。尽管如此，我猜你正试图找到所有与'user1'不是朋友的用户..？在这种情况下，这应该有效：

SELECT username 
FROM users 
WHERE username NOT IN (
SELECT DISTINCT you 
FROM friends 
WHERE friend = 'user1'
UNION ALL 
SELECT DISTINCT friend 
FROM friends 
WHERE you = 'user1') 
AND username <> 'user1'

改进我的查询以提高效率

1 个答案: