How do I decide when to use right joins/left joins

Question

I know the usage of joins, but sometimes I come across such a situation when I am not able to decide which join will be suitable, a left or right.

Here is the query where I am stuck.

    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

I think what you're looking for is to do a LEFT JOIN starting from the main-table to return all records from the main table regardless if they have valid data in the joined ones (as indicated by the top left 2 circles in the graphic)

JOIN's happen in succession, so if you have 4 tables to join, and you always want all the records from your main table, you need to continue LEFT JOIN throughout, for example:

SELECT * FROM main_table
LEFT JOIN sub_table ON main_table.ID = sub_table.main_table_ID
LEFT JOIN sub_sub_table on main_table.ID = sub_sub_table.main_table_ID

If you INNER JOIN the sub_sub_table, it will immediately shrink your result set down even if you did a LEFT JOIN on the sub_table.

Remember, when doing LEFT JOIN, you need to account for NULL values being returned. Because if no record can be joined with the main_table, a LEFT JOIN forces that field to appear regardless and will contain a NULL. INNER JOIN will obviously just "throw away" the row instead because there's no valid link between the two (no corresponding record based on the ID's you've joined)

However, you mention you have a where statement that filters out the rows you're looking for, so your question on the JOIN's are null & void because that is not your real problem. (This is if I understand your comments correctly)

Answer 2

Yes, it depends on the situation you are in.

Why use SQL JOIN?

Answer: Use the SQL JOIN whenever multiple tables must be accessed through an SQL SELECT statement and no results should be returned if there is not a match between the JOINed tables.

Reading this original article on The Code Project will help you a lot: Visual Representation of SQL Joins.

Also check this post: SQL SERVER – Better Performance – LEFT JOIN or NOT IN?.

Find original one at: Difference between JOIN and OUTER JOIN in MySQL.

Answer 3

In two sets:

• Use a full outer join when you want all the results from both sets.

• Use an inner join when you want only the results that appear in both sets.

• Use a left outer join when you want all the results from set a, but if set b has data relevant to some of set a's records, then you also want to use that data in the same query too.

Please refer to the following image: