Video and Stored Procedure: Generate N-length codes with T-SQL and a Tally Table

This is my first attempt at short Transact-SQL videos. I was answering a question on a SQL Server related Facebook group. The question was about generating four-letter codes containing all possible combinations of letters A-Z. There are 26 letters in the english alphabet. Meaning the T-SQL code should generate 26^4 rows, or 456,976 rows.

I immediately thought this is great fit for a Tally Table solution (Read more in this article by Jeff Moden on SQL Server Central: The “Numbers” or “Tally” Table: What it is and how it replaces a loop – SQLServerCentral). And also that it would be a nice fit or a quick demo-video.

So first of all, here’s the video.

But I worked a bit further on the solution, and created a stored procedure which with parameters can create any length strings for the codes and also persist the data in a table instead of returning the rows to the caller.

What I’ve done is generate a string with SQL-code, based on the parameters, and then execute the SQL-code with the system stored procedure sys.sp_executesql.

CREATE OR ALTER PROC dbo.GenerateLetterStrings
(
    @LetterCount TINYINT,
    @ResultTable NVARCHAR(128) = NULL
)
AS
BEGIN
    SET NOCOUNT ON;
    DECLARE @sql NVARCHAR(MAX)
        = N'
WITH alphabet AS (
	SELECT CHAR(ASCII(''A'')+ ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) -1) AS n 
	FROM (VALUES(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1),(1)) t(n)
)SELECT 
	<<selectlist>> AS word
	<<intoclause>>
FROM alphabet AS t0
<<crossjoin>>
'   ;
    DECLARE @SelectList NVARCHAR(MAX) = N't0.n',
            @CrossJoin NVARCHAR(MAX) = N'';
    WITH eight
    AS (SELECT n
        FROM
        (
            VALUES
                (1),
                (1),
                (1),
                (1),
                (1),
                (1),
                (1),
                (1)
        ) t (n) ),
         twofiftysix
    AS (SELECT TOP (@LetterCount - 1)
               ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) AS n
        FROM eight
            CROSS JOIN eight e2
        ORDER BY 1)
    SELECT @CrossJoin = @CrossJoin + CONCAT('CROSS JOIN alphabet as t', n, '
		'),
           @SelectList = @SelectList + CONCAT('+t', n, '.n')
    FROM twofiftysix;
    SET @sql = REPLACE(@sql, N'<<selectlist>>', @SelectList);
    SET @sql = REPLACE(@sql, N'<<crossjoin>>', @CrossJoin);
    SET @sql = REPLACE(@sql, N'<<intoclause>>', COALESCE(N'INTO ' + @ResultTable, N''));
    EXEC sys.sp_executesql @sql;
END;