Left, Right, or Inner Join: Which SQL Join Type Should I Use?

Determining the Appropriate Join Type: Left, Right, or Inner

In data manipulation operations, selecting the correct join type between tables is crucial to retrieve the desired results. Left, right, and inner joins offer distinct behaviors, and selecting the appropriate one for a specific situation can be challenging. Let's clarify the differences and help you determine which join to use:

Inner Join

• Returns rows that have matching values in both joined tables.
• Excludes rows that do not have matching values in both tables.
• Ensures that every row in both tables is represented in the result.

Left Join

• Returns all rows from the left table, regardless of whether there are matching values in the right table.
• Includes matching rows from the right table if they exist.
• If there are no matching values in the right table for a left table row, it returns NULL values for the right table columns.

Right Join

• Returns all rows from the right table, regardless of whether there are matching values in the left table.
• Includes matching rows from the left table if they exist.
• If there are no matching values in the left table for a right table row, it returns NULL values for the left table columns.

Example

Consider the sample query provided:

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

In this query, a left join is used between batchDetails and folderDetails, and a left join is also used between batchDetails and imageDetails. This indicates that we want to return all rows from batchDetails, even if there are no matching rows in folderDetails or imageDetails. Matching rows from folderDetails and imageDetails will be included if available, otherwise NULL values will be returned.

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