How Can I Extract Multiple Values from a Single Column in SQL Server 2005 and Concatenate Them into a Comma-Separated String?

Extract multi-valued columns and concatenate them into comma-separated strings in SQL Server 2005

When retrieving multiple values ​​associated with a single entity in a relational database, it may be necessary to devise innovative methods to integrate these values ​​into a consistent representation. In this case, the challenge is to extract all the aliases associated with a specific user from the UserAliases table and display them in a column as a single comma-separated string.

To do this, we can leverage the power of T-SQL to build a custom function that uses COALESCE (a versatile function that allows us to select the first non-NULL expression from a specified parameter list). This function can be used to iteratively concatenate aliases (separated by commas) to create a string for each user.

Let’s explore how to do it:

We create a function called [dbo].[GetAliasesById] which accepts user ID (@userID) as input parameter and returns varchar(max) data type, which allows enough storage for merged aliases capacity.

In the function body, we declare a variable @output which will act as an accumulator of comma separated strings.

We use a SELECT statement to retrieve the aliases associated with the provided userID from the UserAliases table.

Using a combination of COALESCE and the ' ' operator, we connect the aliases to the @output variable, making sure to insert a comma separator between each alias.

Finally, we return the cumulative value of @output as the result of the function.

To incorporate this function into a query, we can use a combination of SELECT and GROUP BY. The SELECT statement retrieves the UserID and the results of calling the custom function dbo.GetAliasesByID(UserID) (using each unique UserID). The GROUP BY clause ensures that the results are grouped by UserID, allowing aliases to be combined for each user.

By executing this query you will get the desired output showing all aliases for each user in one column separated by commas:

UserID	Aliases
1	MrX, MrY, MrA
2	Abc, Xyz

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