Question

组合2个JOINS时遇到问题。以下两个陈述都有效。我还在2个DB中包含了相关字段。我的问题是： 1）如何将2个语句合并为1个语句？ 2）如何引用我的php中的行来区分结果是来自mmpreID JOIN还是mmparID JOIN？

由于

DB: matchmaker
mmpreID
mmparID

DB: users
uID
ufname
ulname

SELECT matchmaker.mmpreID, matchmaker.mmparID, users.uID, users.ufname, users.ulname 
FROM matchmaker 
LEFT JOIN users ON matchmaker.mmparID = users.uID

SELECT matchmaker.mmpreID, matchmaker.mmparID, users.uID, users.ufname, users.ulname 
FROM matchmaker 
LEFT JOIN users ON matchmaker.mmpreID = users.uID

Answer 1

SELECT m.mmpreID, m.mmparID, u.uID, u.ufname, u.ulname,
    case 
        when m.mmparID = u.uID then 'mmparID'
        when m.mmpreID = u.uID then 'mmpreID'
        else null 
    end as Source
FROM matchmaker m
LEFT JOIN users u ON m.mmparID = u.uID or m.mmpreID = u.uID

<强>更新

SELECT m.mmpreID, m.mmparID,
    upar.uID as upar_UID, 
    upar.ufname as upar_UFNAME, 
    upar.ulname as upar_ULNAME,
    upre.uID as upre_UID, 
    upre.ufname as upre_UFNAME, 
    upre.ulname as upre_ULNAME
FROM matchmaker m
LEFT JOIN users upar ON m.mmparID = upar.uID 
LEFT JOIN users upre ON m.mmpreID = upre.uID

Answer 2

添加UNION和另一列以查看它来自哪里

SELECT matchmaker.mmpreID, matchmaker.mmparID, users.uID, 
users.ufname, users.ulname, "mmpar only" as source
FROM matchmaker 
LEFT JOIN users ON matchmaker.mmparID = users.uID AND matchmaker.mmparID != matchmaker.mmpreID 

UNION

SELECT matchmaker.mmpreID, matchmaker.mmparID, users.uID, 
users.ufname, users.ulname, "mmpre only" as source
FROM matchmaker 
LEFT JOIN users ON matchmaker.mmpreID = users.uID
AND matchmaker.mmparID != matchmaker.mmpreID 

UNION

SELECT matchmaker.mmpreID, matchmaker.mmparID, users.uID, 
users.ufname, users.ulname, "both" as source
FROM matchmaker 
LEFT JOIN users ON matchmaker.mmpreID = users.uID  AND matchmaker.mmparID = matchmaker.mmpreID

修改上述答案，包括3种可能的结果 1. uID仅匹配mmparID 2. uID仅匹配mmpreID 3. uID匹配mmparID和mmpreID

是的，UNIONS效率不高，但有时需要它们

MySQL：结合2个JOINS

2 个答案: