Question

我有一个巨大的MySQL查询，取决于JOIN。

SELECT m.id, l.name as location, CONCAT(u.firstName, " ", u.lastName) AS matchee, u.email AS mEmail, u.description AS description, m.time AS meetingTime
FROM matches AS m
LEFT JOIN locations AS l ON locationID=l.id 
LEFT JOIN users AS u ON (u.id=m.user1ID)
WHERE m.user2ID=2

UNION

SELECT m.id, l.name as location, CONCAT(u.firstName, " ", u.lastName) AS matchee, u.email AS mEmail, u.description AS description, m.time AS meetingTime
FROM matches AS m
LEFT JOIN locations AS l ON locationID=l.id 
LEFT JOIN users AS u ON (u.id=m.user2ID)
WHERE m.user1ID=2

每个子语句的前3行除以UNION是相同的。我如何遵守DRY原则，不重复这三行，并使这个查询更简洁？

Answer 1

尝试这样，应该工作：

SELECT m.id, l.name as location, CONCAT(u.firstName, " ", u.lastName) AS matchee, u.email AS mEmail, u.description AS description, m.time AS meetingTime
FROM matches AS m
LEFT JOIN locations AS l ON locationID=l.id 
LEFT JOIN users AS u 
ON ((u.id=m.user1ID AND m.user2ID=2) OR (u.id=m.user2ID AND  m.user1ID=2))
WHERE (m.user1ID=2 OR m.user2ID=2)

Answer 2

SELECT
  s.id,
  s.location,
  CONCAT(u.firstName, " ", u.lastName) AS matchee,
  u.email AS mEmail,
  u.description AS description,
  s.meetingTime
FROM (
  SELECT
    m.id,
    l.name AS location,
    m.time AS meetingTime,
    CASE m.user1ID when @userID THEN m.user2ID ELSE m.user1ID END AS userID
  FROM matches AS m
  LEFT JOIN locations AS l ON m.locationID = l.id
  WHERE m.user1ID = @userID OR m.user2ID = @userID
) AS s
LEFT JOIN users AS u ON s.userID = u.id

子查询过滤指定用户标识上的matches表，将其连接到locations，从两个表中提取必要的列并准备一个userID列（使用任一{要加入user1ID，{1}}或user2ID，取决于两者中的哪一个不是指定的ID）。外部查询将内部SELECT的结果集连接到users，并拉出由于连接而获得的其余列。

Answer 3

1）我看到您正在匹配用户，并且您保留了与该匹配相关的一些信息。

设计有点复杂。简单的方法就是使匹配中的数据变得多余。通过这种方式，您将始终获得user1ID = 2的匹配。意味着您需要在一个user1ID和user2ID中存储每个匹配2次，在第二个中使用相反的user1ID和user2ID值存储。然而，正如我所说，这是多余的，如果你期望很多记录我不推荐它。

2）其他方法是规范您的设计。创建另一个表matchUser（matchId，userId）（主键是attrs）

SELECT ... FROM matches AS m
LEFT JOIN locations AS l ON locationID=l.id 
LEFT JOIN users AS u ON (u.id=mu2.userId)
INNER JOIN matchUser mu1 ON mu1.userId=2
INNER JOIN matchUser mu2 ON mu2.matchId=mu1.matchId AND mu2.userId != 2

像这样的东西，查询可能是错的我没有测试它，但你会明白的。

如何使用连接更简洁的MySQL查询？

3 个答案: