SQL Server equivalente a la función substring_index en MySQL

Anduvimos buscado por distintos sitios y así de este modo darte la respuesta a tu duda, si tienes dudas deja tu inquietud y respondemos porque estamos para servirte.

Pruebe esta solución basada en T-SQL y XQuery ((root/row)[position() <= sql:variable("@count")]):

Función escalar de T-SQL:

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'' + REPLACE( (SELECT @str AS '*' FOR XML PATH('')) , @delim, N'' ) + N'');

    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

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

Producción:

/*
Result
---------------
www.somewebsite
*/

o

Función con valores de tabla en línea de TSQL:

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'' + REPLACE( (SELECT @str AS '*' FOR XML PATH('')) , @delim, N'' ) + N''))
    )   
    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

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;

Producción:

/*
Value               Result
------------------- ---------------
www.somewebsite.com www.somewebsite
www.yahoo.com       www.yahoo
www.outlook.com     www.outlook
*/

Necesitaba esto recientemente, así que escribí la siguiente función almacenada. Al final, hay un montón de pruebas para asegurarse de que funciona exactamente como lo hace la función MySql (los resultados esperados se copiaron de MySql después de ejecutar las mismas pruebas allí):

-- Function to reproduce the useful functionality of SUBSTRING_INDEX from MySql
CREATE FUNCTION dbo.SUBSTRING_INDEX(@InString  NVARCHAR(Max),
                                    @Delimiter NVARCHAR(Max),
                                    @Count     INT)
RETURNS NVARCHAR(200)
AS
BEGIN
    DECLARE @Pos INT;
    DECLARE @DelimiterOffsets TABLE
    (
         i      INT IDENTITY(1, 1) NOT NULL,
         offset INT NOT NULL
    );

    -- If @Count is zero, we return '' as per spec
    IF @Count = 0
    BEGIN
        RETURN '';
    END;

    DECLARE @OrigLength      INT = LEN(@InString);
    DECLARE @DelimiterLength INT = LEN(@Delimiter);

    -- Prime the pump.
    SET @Pos = Charindex(@Delimiter, @InString, 1);

    -- If the delimiter does not exist in @InString, return the whole string
    IF @Pos = 0
    BEGIN
        RETURN @InString;
    END;

    -- Put all delimiter offsets into @DelimiterOffsets, they get numbered automatically.
    DECLARE @CurrentOffset INT = 0;
    WHILE @Pos > 0
    BEGIN
        SET @CurrentOffset = @Pos;

        INSERT INTO @DelimiterOffsets
                    (offset)
             VALUES (@CurrentOffset);

        SET @Pos = Charindex(@Delimiter, @InString, @CurrentOffset + @DelimiterLength);
    END;

    -- This number is guaranteed to be > 0.
    DECLARE @DelimitersFound INT = (SELECT Count(*) FROM @DelimiterOffsets);

    -- If they requested more delimiters than were found, return the whole string, as per spec.
    IF Abs(@Count) > @DelimitersFound
    BEGIN
        RETURN @InString;
    END;

    DECLARE @StartSubstring INT = 0;
    DECLARE @EndSubstring   INT = @OrigLength;

    -- OK, now return the part they requested
    IF @Count > 0
    BEGIN
        SET @EndSubstring = (SELECT offset 
                               FROM @DelimiterOffsets 
                              WHERE i = @Count);
    END
    ELSE
    BEGIN
        SET @StartSubstring = (SELECT offset + @DelimiterLength 
                                 FROM @DelimiterOffsets 
                                WHERE i = (@DelimitersFound + @Count + 1));
    END;

    RETURN Substring(@InString, @StartSubstring, @EndSubstring);
END; 

Go 

GRANT EXECUTE ON [dbo].SUBSTRING_INDEX TO PUBLIC;

-- Tests
DECLARE @TestResults TABLE (i int, answer nVarChar(MAX), expected nVarChar(MAX));

insert into @TestResults
select * from  
(
    (SELECT  1 as i, [dbo].SUBSTRING_INDEX(N'www.somewebsite.com', N'.', 2)    as r, 'www.somewebsite'     as e) UNION
    (SELECT  2 as i, [dbo].SUBSTRING_INDEX(N'www.yahoo.com', N'.', 2)          as r, 'www.yahoo'           as e) UNION
    (SELECT  3 as i, [dbo].SUBSTRING_INDEX(N'www.outlook.com', N'.', 2)        as r, 'www.outlook'         as e) UNION
    (SELECT  4 as i, [dbo].SUBSTRING_INDEX(N'www.somewebsite.com', N'.', -2)   as r, 'somewebsite.com'     as e) UNION
    (SELECT  5 as i, [dbo].SUBSTRING_INDEX(N'www.yahoo.com', N'.', -2)         as r, 'yahoo.com'           as e) UNION
    (SELECT  6 as i, [dbo].SUBSTRING_INDEX(N'www.outlook.com', N'.', -2)       as r, 'outlook.com'         as e) UNION
    (select  7 as i, [dbo].SUBSTRING_INDEX('hi.you.com','.',2)                 as r, 'hi.you'              as e) UNION
    (select  8 as i, [dbo].SUBSTRING_INDEX('hi.you.com','.',-1)                as r, 'com'                 as e) UNION
    (select  9 as i, [dbo].SUBSTRING_INDEX(N'prueba','ue',1)                   as r, 'pr'                  as e) UNION
    (select 10 as i, [dbo].SUBSTRING_INDEX(N'prueba','ue',-1)                  as r, 'ba'                  as e) UNION
    (select 11 as i, [dbo].SUBSTRING_INDEX(N'prueba','ue',0)                   as r, ''                    as e) UNION
    (SELECT 12 as i, [dbo].SUBSTRING_INDEX(N'wwwxxxoutlookxxxcom', N'xxx', 2)  as r, 'wwwxxxoutlook'       as e) UNION
    (SELECT 13 as i, [dbo].SUBSTRING_INDEX(N'wwwxxxoutlookxxxcom', N'xxx', -2) as r, 'outlookxxxcom'       as e) UNION
    (SELECT 14 as i, [dbo].SUBSTRING_INDEX(N'wwwxxxoutlookxxxcom', N'xxx', 5)  as r, 'wwwxxxoutlookxxxcom' as e) UNION
    (SELECT 15 as i, [dbo].SUBSTRING_INDEX(N'wwwxxxoutlookxxxcom', N'xxx', -5) as r, 'wwwxxxoutlookxxxcom' as e)
) as results;

select tr.i,
       tr.answer,
       tr.expected,
       CASE WHEN tr.answer = tr.expected THEN 'Test Succeeded' ELSE 'Test Failed' END testState
  from @TestResults tr
 order by i;

Aquí hay una versión inspirada en la respuesta de Bogdan Sahlean que usa la funcionalidad XML de SQL Server para realizar el análisis y la combinación:

CREATE FUNCTION dbo.SUBSTRING_INDEX(@InString  NVARCHAR(Max),
                                    @Delimiter NVARCHAR(Max),
                                    @Count     INT)
RETURNS NVARCHAR(200)
AS
BEGIN
    -- If @Count is zero, we return '' as per spec
    IF @Count = 0
    BEGIN
        RETURN '';
    END;

    -- First we let the XML parser break up the string by @Delimiter.
    -- Each parsed value will be [text].
    DECLARE @XmlSourceString XML = (select N'' + REPLACE( (SELECT @InString AS '*' FOR XML PATH('')) , @Delimiter, N'' ) + N'');

    -- This will contain the final requested string.
    DECLARE @Results nVarChar(MAX);

    ;WITH Pieces(RowNumber, Piece) as
    (
        -- Take each node in @XmlSourceString, and return it with row numbers
        -- which will identify each piece and give us a handle to change the
        -- order, depending on the direction of search.
        SELECT  row_number() over(order by x.XmlCol) as RowNumber,
                @Delimiter + x.XmlCol.value(N'(text())[1]', N'NVARCHAR(MAX)') AS '*'
          FROM  @XmlSourceString.nodes(N'(piece)') x(XmlCol)
    ), orderedPieces(RowNumber, Piece) as
    (
        -- Order the pieces normally or reversed depending on whether they want
        -- the first @Count pieces or the last @Count pieces.
        select TOP (ABS(@Count)) 
               RowNumber, 
               Piece
          from Pieces
         ORDER BY CASE WHEN @Count < 0 THEN RowNumber END DESC ,
                  CASE WHEN @Count > 0 THEN RowNumber END ASC
    ), combinedPieces(result) as
    (
        -- Now combine the pieces back together, ordering them by
        -- the original order.  There will always
        -- be an extra @Delimiter on the front of the string.
        select CAST(Piece AS VARCHAR(100))
          from OrderedPieces
         order by RowNumber
           FOR XML PATH(N'')
    )
    -- Finally, strip off the extra delimiter using STUFF and store the string in @Results.
    select @Results = STUFF(result, 1, LEN(@Delimiter), '') from combinedPieces;

    return @Results;
END;

La ejecución de las pruebas produce esto:

i  answer              expected             testState
1  www.somewebsite     www.somewebsite      Test Succeeded
2  www.yahoo           www.yahoo            Test Succeeded
3  www.outlook         www.outlook          Test Succeeded
4  somewebsite.com     somewebsite.com      Test Succeeded
5  yahoo.com           yahoo.com            Test Succeeded
6  outlook.com         outlook.com          Test Succeeded
7  hi.you              hi.you               Test Succeeded
8  com                 com                  Test Succeeded
9  pr                  pr                   Test Succeeded
10 ba                  ba                   Test Succeeded
11                                          Test Succeeded
12 wwwxxxoutlook       wwwxxxoutlook        Test Succeeded
13 outlookxxxcom       outlookxxxcom        Test Succeeded
14 wwwxxxoutlookxxxcom wwwxxxoutlookxxxcom  Test Succeeded
15 wwwxxxoutlookxxxcom wwwxxxoutlookxxxcom  Test Succeeded

Si crees que te ha resultado de utilidad este artículo, sería de mucha ayuda si lo compartes con otros entusiastas de la programación de este modo contrubuyes a extender nuestro contenido.

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