Updating row columns from a comma-separated column from the same table, Sql Server

Language: Layout:

CREATE FUNCTION [dbo].[DelimitedSplit8K]
--===== Define I/O parameters
        (@pString VARCHAR(8000), @pDelimiter CHAR(1))
--WARNING!!! DO NOT USE MAX DATA-TYPES HERE!  IT WILL KILL PERFORMANCE!
RETURNS TABLE WITH SCHEMABINDING AS
 RETURN
--===== "Inline" CTE Driven "Tally Table" produces values from 1 up to 10,000...
     -- enough to cover VARCHAR(8000)
  WITH E1(N) AS (
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
                ),                          --10E+1 or 10 rows
       E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E+2 or 100 rows
       E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E+4 or 10,000 rows max
 cteTally(N) AS (--==== This provides the "base" CTE and limits the number of rows right up front
                     -- for both a performance gain and prevention of accidental "overruns"
                 SELECT TOP (ISNULL(DATALENGTH(@pString),0)) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
                ),
cteStart(N1) AS (--==== This returns N+1 (starting position of each "element" just once for each delimiter)
                 SELECT 1 UNION ALL
                 SELECT t.N+1 FROM cteTally t WHERE SUBSTRING(@pString,t.N,1) = @pDelimiter
                ),
cteLen(N1,L1) AS(--==== Return start and length (for use in substring)
                 SELECT s.N1,
                        ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000)
                   FROM cteStart s
                )
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
 SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY l.N1),
        Item       = SUBSTRING(@pString, l.N1, l.L1)
   FROM cteLen l
;
GO

CREATE TABLE _Table 
(
    [Pat] NVARCHAR(8), 
    [Codes] NVARCHAR(50), 
    [C1] NVARCHAR(6), 
    [C2] NVARCHAR(6), 
    [C3] NVARCHAR(6), 
    [C4] NVARCHAR(6), 
    [C5] NVARCHAR(6)
);
GO

INSERT INTO _Table ([Pat], [Codes], [C1], [C2], [C3], [C4], [C5])
VALUES
    ('Pat1', 'U212,Y973,Y982', null, null, null, null, null),
    ('Pat2', 'M653', null, null, null, null, null), 
    ('Pat3', 'U212,Y973,Y983,Z924,Z926', null, null, null, null, null);
GO

;WITH CTE1 AS
(
    SELECT Pat, Codes, ItemNumber, Item
    FROM _Table
    CROSS APPLY [dbo].[DelimitedSplit8K](Codes, ',')
), CTE2 AS
(
    SELECT  Pat, 
            Codes, 
            MAX(CASE WHEN ItemNumber = 1 THEN Item END) As V1, 
            MAX(CASE WHEN ItemNumber = 2 THEN Item END) As V2, 
            MAX(CASE WHEN ItemNumber = 3 THEN Item END) As V3, 
            MAX(CASE WHEN ItemNumber = 4 THEN Item END) As V4, 
            MAX(CASE WHEN ItemNumber = 5 THEN Item END) As V5
    FROM CTE1
    GROUP BY [Pat], [Codes]
)

UPDATE t
SET C1 = V1,
    C2 = V2,
    C3 = V3,
    C4 = V4,
    C5 = V5
FROM _Table t
JOIN CTE2 ON t.Pat = CTE2.Pat

SELECT *
FROM _Table

View schema
Execution time: 0,05 sec, rows selected: 3, rows affected: 6, absolute service time: 0,19 sec