How to Replicate MySQL's SUBSTRING_INDEX Function in SQL Server?

SQL Server equivalent of the MySQL substring_index function

When migrating queries from MySQL to SQL Server, it's often necessary to find SQL Server equivalents for commonly used MySQL functions.

MySQL's substring_index() function

MySQL's SUBSTRING_INDEX() function retrieves a substring from a given string based on the occurrence of a specified delimiter. Its syntax is:

SUBSTRING_INDEX(str, delim, count)

For instance:

SELECT SUBSTRING_INDEX('www.somewebsite.com','.',2);

This would return 'www.somewebsite'.

Equivalent in SQL Server T-SQL

One viable option in T-SQL for replicating SUBSTRING_INDEX()'s functionality is a combination of XQuery and a scalar function:

CREATE FUNCTION dbo.SUBSTRING_INDEX
(
    @str NVARCHAR(4000),
    @delim NVARCHAR(1),
    @count INT
)
RETURNS NVARCHAR(4000)
WITH SCHEMABINDING
BEGIN
    DECLARE @XmlSourceString XML;
    SET @XmlSourceString = (SELECT N'<root><row>' + REPLACE( (SELECT @str AS '*' FOR XML PATH('')) , @delim, N'</row><row>' ) + N'</row></root>');

    RETURN STUFF
    (
        ((
            SELECT  @delim + x.XmlCol.value(N'(text())[1]', N'NVARCHAR(4000)') AS '*'
            FROM    @XmlSourceString.nodes(N'(root/row)[position() <= sql:variable("@count")]') x(XmlCol)
            FOR XML PATH(N''), TYPE
        ).value(N'.', N'NVARCHAR(4000)')), 
        1, 1, N''
    );
END
GO

Example usage:

SELECT dbo.SUBSTRING_INDEX(N'www.somewebsite.com', N'.', 2) AS Result;

Output:

Result
---------------
www.somewebsite

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