[英]I have an SQL query which links 3 tables using UNION:
$sql ="(SELECT Drive.DriveID,Ram.Memory from Drive,Ram where Drive.DriveID = Ram.RamID) UNION
(SELECT Drive.DriveID,External.Memory from Drive, External where Drive.DriveID = External.ExtID)";
假设我也想获得Ram.Name。 我该怎么做呢? 如果我在第一个SELECT语句中使用Ram.Name,它将不会产生正确的结果。
解决这个问题的方法有哪些? 我想用UNION来做。
怎么样:
SELECT Drive.DriveID,Ram.Memory,Ram.Name
FROM Drive,Ram WHERE Drive.DriveID = Ram.RamID
UNION
SELECT Drive.DriveID,External.Memory,NULL
FROM Drive, External WHERE Drive.DriveID = External.ExtID
对于第二个选择的所有结果,这将在第四列中为NULL
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