从表中连接列并合并具有相同ID的行

Question

我正在研究一个SQL查询，而且我在连接中完全迷失了自己。希望你能够提供帮助：）

我的问题是，单独表中的行不会合并为一行，它们都具有相同的ID。很长一段时间，加入对我来说很困惑，所以任何帮助都会受到高度赞赏。

我有以下表格：

Students
id

StudentCredits
sum_credits

StudentMandatoryCredits
branch_credits
program_credits

我正在尝试创建一个查询，它会给我这样的行：

id         - sum_credits - branch_credits - program_credits
another_id - sum_credits - branch_credits - program_credits
a_third_id - sum_credits - branch_credits - program_credits

这是我能想到的最好的：

id - sum_credits - branch_credits -     [empty]
id - sum_credits -     [empty]    - program_credits

这是我的SQL语句：

SELECT S.id, SC.sum_credits, SMC.branch_credits, SMC.program_credits
FROM Students S
LEFT JOIN StudentCredits SC ON S.id = SC.id
LEFT JOIN StudentMandatoryCredits SMC ON S.id = SMC.id
ORDER BY id

Answer 1

尝试使用：

SELECT S.id, SUM(SC.sum_credits), SUM(SMC.branch_credits), SUM(SMC.program_credits)
FROM Students S
LEFT JOIN StudentCredits SC ON S.id = SC.id
LEFT JOIN StudentMandatoryCredits SMC ON S.id = SMC.id
GROUP BY S.id
ORDER BY S.id