SQL Server Equivalent of substring_index Function in MySQL

The substring_index function in MySQL is used to extract a substring from a specified string before a given number of occurrences of a delimiter. To achieve similar functionality in SQL Server 2012, you can employ one of the following approaches:

T-SQL Scalar Function:

CREATE FUNCTION dbo.SUBSTRING_INDEX
(
    @str NVARCHAR(4000),
    @delim NVARCHAR(1),
    @count INT
)
RETURNS NVARCHAR(4000)
WITH SCHEMABINDING
BEGIN
    -- Convert the string to an XML document for processing
    DECLARE @XmlSourceString XML;
    SET @XmlSourceString = (SELECT N'<root><row>'   REPLACE( (SELECT @str AS '*' FOR XML PATH('')) , @delim, N'</row><row>' )   N'</row></root>');

    -- Extract the desired substring using XQuery
    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;

T-SQL Inline Table-Valued Function:

CREATE FUNCTION dbo.SUBSTRING_INDEX
(
    @str NVARCHAR(4000),
    @delim NVARCHAR(1),
    @count INT
)
RETURNS TABLE
AS 
RETURN
    WITH Base
    AS 
    (
        SELECT XmlSourceString = CONVERT(XML, (SELECT N'<root><row>'   REPLACE( (SELECT @str AS '*' FOR XML PATH('')) , @delim, N'</row><row>' )   N'</row></root>'))
    )   
    SELECT STUFF
    (
        ((
            SELECT  @delim   x.XmlCol.value(N'(text())[1]', N'NVARCHAR(4000)') AS '*'
            FROM    Base b 
            CROSS APPLY b.XmlSourceString.nodes(N'(root/row)[position() <= sql:variable("@count")]') x(XmlCol)
            FOR XML PATH(N''), TYPE
        ).value(N'.', N'NVARCHAR(4000)')), 
        1, 1, N''
    ) AS Result;
GO

-- Example usage
SELECT  *
FROM    (
    SELECT N'www.somewebsite.com' UNION ALL 
    SELECT N'www.yahoo.com' UNION ALL 
    SELECT N'www.outlook.com'
) a(Value)
CROSS APPLY dbo.SUBSTRING_INDEX(a.Value, N'.', 2) b;

These functions allow you to replicate the behavior of the MySQL substring_index function in SQL Server, enabling you to portably perform substring extraction tasks.