How to Concatenate Grouped Values in SQL Server Without GROUP_CONCAT?

SQL Server group value splicing: no GROUP_CONCAT required

In database management, it is often necessary to merge values ​​from different rows within a group. For example, in a table, a single ID is associated with multiple values:

Id	Value
1	'A'
1	'B'
2	'C'

The goal is to convert the data into a concatenated string of each ID and its associated value:

Id	Value
1	'AB'
2	'C'

Achieving this in SQL Server may seem difficult since it does not have MySQL's GROUP_CONCAT function. However, SQL Server can easily solve this problem using user-defined aggregate functions (UDFs).

In order to create a valid string concatenation UDF, let us analyze the solutions provided in the duplicate question "How to concatenate strings using GROUP BY in SQL Server?" The core part is an aggregate function that accepts two string parameters, concatenates them, and returns the result. By encapsulating this operation in a UDF, we can use it in queries.

The following is the code snippet to create the UDF:

CREATE FUNCTION [dbo].[StringConcat](@Val1 VARCHAR(MAX), @Val2 VARCHAR(MAX))
RETURNS VARCHAR(MAX) AS
BEGIN
    RETURN COALESCE(@Val1 + @Val2, '')
END;

Now we can use this UDF in a simple query:

SELECT Id, StringConcat(Value, '') AS Value
FROM YourTable
GROUP BY Id;

The final query plan shows the efficiency of this approach. With the help of user-defined aggregate functions, we can easily concatenate the values ​​of grouped rows, effectively solving database challenges.

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