mysql - 我如何决定何时使用右连接/左连接或内连接或者如何确定哪个表在哪一侧？

Question

我知道连接的用法，但有时我会遇到这样的情况，即我无法确定哪个 连接 适合，左或右。

这是我卡住的查询。

    SELECT  count(ImageId) as [IndividualRemaining],
                userMaster.empName AS ID#,
                CONVERT(DATETIME, folderDetails.folderName, 101) AS FolderDate,
                batchDetails.batchName AS Batch#,
                Client=@ClientName,
                TotalInloaded = IsNull(@TotalInloaded,0),
                PendingUnassigned = @PendingUnassigned,
                InloadedAssigned =     IsNull(@TotalAssigned,0),
                TotalProcessed = @TotalProcessed,
                Remaining = @Remaining
        FROM
                batchDetails
                    Left JOIN  folderDetails ON batchDetails.folderId = folderDetails.folderId
                    Left JOIN  imageDetails ON batchDetails.batchId = imageDetails.batchId
                    Left JOIN  userMaster ON imageDetails.assignedToUser = userMaster.userId

        WHERE   folderDetails.ClientId =@ClientID and verifyflag='n'
                and folderDetails.FolderName IN (SELECT convert(VARCHAR,Value) FROM dbo.Split(@Output,','))
                and userMaster.empName <> 'unused'

        GROUP BY userMaster.empName, folderDetails.folderName, batchDetails.batchName

        Order BY folderDetails.Foldername asc

Answer 1

是的，这取决于你所处的情况。

为什么要使用 SQL JOIN？

答案:当必须通过 SQL SELECT 语句访问多个表时使用 SQL JOIN，如果 JOIN 的表之间不匹配，则不应返回任何结果。

在 The Code Project 上阅读这篇原创文章对你有很大帮助:Visual Representation of SQL Joins .

另请查看此帖子:SQL SERVER – Better Performance – LEFT JOIN or NOT IN? .

在 Difference between JOIN and OUTER JOIN in MySQL 上查找原件.