How to Convert UTF-8 Text to ISO 8859-1 in SQL Server?

Convert Text Value from UTF-8 to ISO 8859-1 in SQL Server

Problem: You have a column in SQL Server with UTF-8 encoding and you need to convert and save the text in ISO 8859-1 encoding.

Solution: To achieve this conversion, you can utilize a custom function. The following code provides an implementation:

CREATE FUNCTION dbo.DecodeUTF8String (@value varchar(max))
RETURNS nvarchar(max)
AS
BEGIN
    -- Check if the string is ASCII or null, as no conversion is needed.
    IF (@value IS NULL
        OR @value NOT LIKE '%[^ -~]%' COLLATE Latin1_General_BIN
    )
        RETURN @value;

    DECLARE @result nvarchar(max);

    -- Split the UTF-8 string into octets and calculate their codepoints.
    WITH octets AS
    (
        SELECT position, highorderones, partialcodepoint
        FROM numbers a
        CROSS APPLY (SELECT octet = ASCII(SUBSTRING(@value, position, 1))) b
        CROSS APPLY (SELECT highorderones = 8 - FLOOR(LOG( ~CONVERT(tinyint, octet) * 2 + 1)/LOG(2))) c
        CROSS APPLY (SELECT databits = 7 - highorderones) d
        CROSS APPLY (SELECT partialcodepoint = octet % POWER(2, databits)) e
    ),
    codepoints AS
    (
        SELECT position, codepoint
        FROM
        (
            SELECT position, highorderones, partialcodepoint
            FROM octets
            WHERE highorderones <> 1
        ) lead
        CROSS APPLY (SELECT sequencelength = CASE WHEN highorderones in (1,2,3,4) THEN highorderones ELSE 1 END) b
        CROSS APPLY (SELECT endposition = position + sequencelength - 1) c
        CROSS APPLY
        (
            SELECT codepoint = SUM(POWER(2, shiftleft) * partialcodepoint)
            FROM octets
            CROSS APPLY (SELECT shiftleft = 6 * (endposition - position)) b
            WHERE position BETWEEN lead.position AND endposition
        ) d
    )
    -- Concatenate the codepoints into a Unicode string.
    SELECT @result = CONVERT(xml,
        (
            SELECT NCHAR(codepoint)
            FROM codepoints
            ORDER BY position
            FOR XML PATH('')
        )).value('.', 'nvarchar(max)');

    RETURN @result;
END
GO

Usage: To apply the conversion to a column named "Column1," you can use the following query:

SELECT *, NewColumn1 = dbo.DecodeUTF8String(Column1)
FROM Table1
WHERE Column1 <> dbo.DecodeUTF8String(Column1)

This query will create a new column, "NewColumn1," which contains the UTF-8 text converted to ISO 8859-1 encoding.

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