MySQL 中substring_index 函數的SQL Server 等效項

MySQL 中的substring_index 函數用於從給定字串之前的指定字元串中提取子字串分隔符號出現的次數。若要在 SQL Server 2012 中實作類似的功能，您可以採用下列方法之一：

T-SQL 標量函數：

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 內聯表值函數:

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;

這些函數可讓您複製SQL Server 中MySQL substring_index 函數的行為，讓您能夠便攜式執行子字串提取任務.