How to Get Distinct Values Using STRING_AGG in SQL Server?

Distinct Values using STRING_AGG in SQL Server

The STRING_AGG function in SQL Server 2017 allows you to concatenate values into a single string. However, it can return duplicate values when concatenating a field with multiple occurrences.

Consider the following query:

SELECT 
    ProjectID,
    STRING_AGG( newID.value, ',') WITHIN GROUP (ORDER BY newID.value) AS NewField
FROM
    [dbo].[Data] WITH(NOLOCK)  
CROSS APPLY 
    STRING_SPLIT([bID],';') AS newID  
WHERE 
    newID.value IN ('O95833', 'Q96NY7-2')  
GROUP BY 
    ProjectID
ORDER BY 
    ProjectID

This query returns the concatenated values, but duplicates are included:

ProjectID	NewField
2	O95833,O95833,O95833,Q96NY7-2,Q96NY7-2,Q96NY7-2
4	Q96NY7-2,Q96NY7-2

To retrieve only unique values, use the DISTINCT keyword in a subquery:

SELECT 
 ProjectID
,STRING_AGG(value, ',') WITHIN GROUP (ORDER BY value) AS 
NewField
FROM (
    SELECT DISTINCT 
      ProjectID
    , newId.value 
    FROM [dbo].[Data] WITH (NOLOCK)  
    CROSS APPLY STRING_SPLIT([bID],';') AS newId  
    WHERE newId.value IN (   'O95833' , 'Q96NY7-2'  )  
) x
GROUP BY ProjectID
ORDER BY ProjectID

This subquery removes duplicates before applying the STRING_AGG function. The final result is:

ProjectID	NewField
2	O95833, Q96NY7-2
4	Q96NY7-2

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