What is the equivalent of String.Join on TSQL? [duplicate]

This should do the trick:

DECLARE @Result VARCHAR(MAX);

SELECT
    @Result = CASE
        WHEN @Result IS NULL
        THEN T.TextColumn
        ELSE @Result + ';' + T.TextColumn
    END
FROM
    SomeTable AS T
WHERE
    T.SomeOtherColumn = 'Y';

SELECT @Result