从另一个表中选择具有两个不同外键的同一列

Question

我觉得我接近我的解决方案，但并不完全在那里。我试图选择testRun.build两次，

首次基于当前运行（Results.build_fk）并基于最后一次运行所在的测试通过（Results.lastPass）。

这是迄今为止我选择的声明，我觉得应该工作（because of this answer），但它告诉我“子查询返回多于一行”。

SELECT Results.build_fk, Results.testCaseID, Results.pass, Results.time, 

(SELECT testRun.build 

    FROM testRun 

INNER JOIN Results r1 ON r1.build_fk = testRun.runID 

) AS Last_build, 

(SELECT testRun.build 

    FROM testRun 

    INNER JOIN Results r2 ON r2.lastPass = testRun.runID 
) 
    AS Current_Build 

FROM Results 

    INNER JOIN testCases AS t1 ON t1.testCaseID = Results.testCaseID 

    ORDER BY build_fk DESC, testCaseID ASC; 

这里是样本表，样本数据：

Results 
(`build_fk`, `testCaseID`, `pass`, `time`, `lastPass`) 
(1132, 200, 0, {some timestamp}, 1132) 
(1133, 200, 0, {some timestamp}, 1132) 
(1134, 200, 1, {some timestamp}, 1134) 
(1132, 210, 0, {some timestamp}, 1132) 
(1133, 210, 0, {some timestamp}, 1132) 
(1134, 210, 1, {some timestamp}, 1134) 

testRun 
(`runID`, `build`) 
(1132, 'build-1.0') 
(1133, 'build-1.1') 
(1134, 'build-1.2') 

对不起世界上没有SQLFiddle，我不能让它正确地创建表。

Answer 1

我想在子查询的附加连接是不必要的：

SELECT r.build_fk, r.testCaseID, r.pass, r.time, 
     (SELECT tr.build 
     FROM testRun tr 
     WHERE r.build_fk = tr.runID 
     ) AS Last_build, 
     (SELECT tr.build 
     FROM testRun tr 
     WHERE r.lastPass = tr.runID 
     ) AS Current_Build 
FROM Results r INNER JOIN 
    testCases t1 
    ON t1.testCaseID = r.testCaseID 
ORDER BY build_fk DESC, testCaseID ASC; 

你想要的子查询（使用r表的别名在这个版本）来引用外部查询。这被称为相关子查询。