select distinct code from @T

;WITH Results AS (
    SELECT id, code
        ,rn=ROW_NUMBER() OVER (PARTITION BY code ORDER BY id)
    FROM @T)
SELECT id, code
FROM Results
WHERE rn=1

;WITH Results AS (
    SELECT id, code
        ,rn=ROW_NUMBER() OVER (PARTITION BY code ORDER BY id)
    FROM @T)
SELECT id, code
FROM Results
WHERE rn=1

SELECT MIN(id) AS id, code
FROM @T
GROUP BY code;

IF object_id('tempdb..#T') IS NOT NULL
BEGIN
    DROP TABLE #T;
END;

SELECT TOP 1000000 IDENTITY(INT,1,1) AS id, 
CAST((ABS(CHECKSUM(NEWID())) % 100) + 1 AS VARCHAR(3)) AS code
INTO #T
FROM master.dbo.syscolumns sc1, master.dbo.syscolumns sc2, master.dbo.syscolumns sc3;

DECLARE @HOLDER INT, @HOLDER2 VARCHAR(3);

PRINT 'ROWNUMBER';
SET STATISTICS IO, TIME ON;

;WITH Results AS (
    SELECT id, code
        ,rn=ROW_NUMBER() OVER (PARTITION BY code ORDER BY id)
    FROM #T)
SELECT @HOLDER = id, @HOLDER2 = code
FROM Results
WHERE rn=1;

SET STATISTICS IO, TIME OFF;

PRINT 'MIN';
SET STATISTICS IO, TIME ON;

SELECT @HOLDER = MIN(id), @HOLDER2 = code
FROM #T
GROUP BY code;

SET STATISTICS IO, TIME OFF;

ROWNUMBER
Table '#T'. Scan count 5, logical reads 2341, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 4797 ms,  elapsed time = 2547 ms.
MIN
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table '#T'. Scan count 5, logical reads 2341, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 626 ms,  elapsed time = 164 ms.

IF object_id('tempdb..#T') IS NOT NULL
BEGIN
    DROP TABLE #T;
END;

SELECT TOP 1000000 IDENTITY(INT,1,1) AS id, 
CAST((ABS(CHECKSUM(NEWID())) % 100) + 1 AS VARCHAR(3)) AS code
INTO #T
FROM master.dbo.syscolumns sc1, master.dbo.syscolumns sc2, master.dbo.syscolumns sc3;

CREATE CLUSTERED INDEX cl_t_id ON #T(id);
CREATE NONCLUSTERED INDEX nc_t_code ON #T(code);

DECLARE @HOLDER INT, @HOLDER2 VARCHAR(3);

PRINT 'ROWNUMBER';
SET STATISTICS IO, TIME ON;

;WITH Results AS (
    SELECT id, code
        ,rn=ROW_NUMBER() OVER (PARTITION BY code ORDER BY id)
    FROM #T)
SELECT @HOLDER = id, @HOLDER2 = code
FROM Results
WHERE rn=1;

SET STATISTICS IO, TIME OFF;

PRINT 'MIN';
SET STATISTICS IO, TIME ON;

SELECT @HOLDER = MIN(id), @HOLDER2 = code
FROM #T
GROUP BY code;

SET STATISTICS IO, TIME OFF;

ROWNUMBER
Table '#T'. Scan count 1, logical reads 1979, physical reads 0, read-ahead reads 15, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 422 ms,  elapsed time = 409 ms.
MIN
Table '#T'. Scan count 1, logical reads 1979, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 391 ms,  elapsed time = 402 ms.