


How to Concatenate Grouped Values in SQL Server Using a User-Defined Aggregate Function?
SQL Server String Aggregation with User-Defined Aggregate Functions
Need to concatenate strings within groups in SQL Server, similar to MySQL's GROUP_CONCAT
? SQL Server 2005 and later versions don't have a built-in equivalent, requiring a more advanced solution. A user-defined aggregate function (UDAF) provides an effective approach.
Consider this sample data:
Id | Value |
---|---|
1 | 'A' |
1 | 'B' |
2 | 'C' |
The goal is to achieve this output:
Id | Value |
---|---|
1 | 'AB' |
2 | 'C' |
Creating the UDAF
The following code defines a UDAF named AggregateConcat
to perform the string concatenation within each group:
CREATE FUNCTION AggregateConcat (@id INT, @value VARCHAR(MAX)) RETURNS VARCHAR(MAX) AS BEGIN DECLARE @result VARCHAR(MAX) = ''; SELECT @result = @result + Value FROM TABLE_NAME WHERE Id = @id; RETURN @result; END;
Using the UDAF
This function can be used in a SELECT
statement like any other aggregate function:
SELECT Id, dbo.AggregateConcat(Id, Value) AS ConcatenatedValue FROM TABLE_NAME GROUP BY Id;
This query groups the data by Id
and applies the AggregateConcat
function to concatenate the Value
column for each group, producing the desired result. Remember to replace TABLE_NAME
with your actual table name. This simplified version avoids the less efficient cursor-based approach. For very large datasets, consider alternative, more performant methods like using STRING_AGG
(available in SQL Server 2017 and later).
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