How to Join Comma-Delimited Data Columns from Two Tables?

Joining Comma-Delimited Data Columns

Data tables often contain comma-separated values within cells. Extracting and normalizing these values can be crucial for further data analysis. In this article, we will explore techniques to join comma-delimited data in two tables and present the results in a consolidated format.

Consider the following two tables:

Table 1 (T1)

col1	col2
C1	john
C2	alex
C3	piers
C4	sara

Table 2 (T2)

col1	col2
R1	C1,C2,C4
R2	C3,C4
R3	C1,C4

Desired Result:

col1	col2
R1	john,alex,sara
R2	piers,sara
R3	john,sara

Solutions:

Normalization:

The ideal solution is to normalize Table 2 by splitting the comma-separated values into a separate relation. This would create a new table with two columns: col1 and col2, where each row represents a value previously stored in the col2 column of Table 2.

Join with Subquery (FOR XML PATH):

If normalization is not feasible, we can use a subquery along with the FOR XML PATH clause to retrieve the comma-separated values as a single string.

SELECT col1, 
(
  SELECT ', '+t1.col2
  FROM t1
  WHERE ','+t2.col2+',' LIKE '%,'+CAST(t1.col1 AS VARCHAR(10))+',%'
  FOR XML PATH(''), TYPE
).value('substring(text()[1], 3)', 'VARCHAR(MAX)') AS col2
FROM t2;

Join with Split Function:

Alternatively, we can create a user-defined function called Split to extract individual values from the comma-delimited string.

CREATE FUNCTION [dbo].[Split](@String VARCHAR(MAX), @Delimiter CHAR(1))
RETURNS @temptable TABLE (items VARCHAR(MAX))
AS
BEGIN
    DECLARE @idx INT;
    DECLARE @slice VARCHAR(8000);

    SELECT @idx = 1;
    IF LEN(@String) < 1 OR @String IS NULL RETURN;

    WHILE @idx != 0
    BEGIN
        SET @idx = CHARINDEX(@Delimiter, @String);
        IF @idx != 0
            SET @slice = LEFT(@String, @idx - 1);
        ELSE
            SET @slice = @String;

        IF (LEN(@slice) > 0)
            INSERT INTO @temptable(Items) VALUES (@slice);

        SET @String = RIGHT(@String, LEN(@String) - @idx);
        IF LEN(@String) = 0 BREAK;
    END
RETURN;
END;

Using the Split function, we can either store the individual values in separate rows or concatenate them back into a comma-separated string.

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