Demo, Sql Server - rextester

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CREATE TABLE Marks_Entry
    ([id] int, [std_id] int, [sub_id] int, [ASG_MARK] varchar(2), [TH_MARKS] varchar(2), [PR_MARCHAR] varchar(2))
;
    
INSERT INTO Marks_Entry
    ([id], [std_id], [sub_id], [ASG_MARK], [TH_MARKS], [PR_MARCHAR])
VALUES
    (1, 123, 1, '20', '50', '30'),
    (2, 123, 2, '18', '40', '20'),
    (3, 123, 3, '20', 'AB', '30'),
    (4, 123, 4, 'AB', '40', '20'),
    (5, 123, 5, 'AB', 'AB', 'AB'),
    (6, 123, 6, '0', '0', '0')
;

UPDATE Marks_Entry
SET    ASG_MARK = NULL
WHERE  ASG_MARK = 'AB'

UPDATE Marks_Entry
SET    TH_MARKS = NULL
WHERE  TH_MARKS = 'AB'

UPDATE Marks_Entry
SET    PR_MARCHAR = NULL
WHERE  PR_MARCHAR = 'AB'

ALTER TABLE Marks_Entry
  ALTER COLUMN ASG_MARK DECIMAL(10, 2)

ALTER TABLE Marks_Entry
  ALTER COLUMN TH_MARKS DECIMAL(10, 2)

ALTER TABLE Marks_Entry
  ALTER COLUMN PR_MARCHAR DECIMAL(10, 2)

SELECT id,
       STD_ID,
       SUB_ID,
       ASG_MARK = COALESCE(Cast(ASG_MARK as varchar(100)),'AB'),
       TH_MARKS = COALESCE(Cast(TH_MARKS as varchar(100)),'AB'),
       PR_MARCHAR = COALESCE(Cast(PR_MARCHAR as varchar(100)),'AB'),
       TOTAL = CASE
                 WHEN ASG_MARK IS NULL AND TH_MARKS IS NULL AND PR_MARCHAR IS NULL THEN 'AB'
                 ELSE Cast(COALESCE(ASG_MARK, 0) + COALESCE(TH_MARKS, 0) + COALESCE(PR_MARCHAR, 0) AS VARCHAR(100))
               END,
       GRADE = CASE
                 WHEN orgTotal BETWEEN 91 AND 100 THEN 'A1'
                 WHEN orgTotal BETWEEN 81 AND 90 THEN 'B1'
                 WHEN orgTotal BETWEEN 71 AND 80 THEN 'C1'
                 WHEN orgTotal BETWEEN 61 AND 70 THEN 'D1'
                 WHEN orgTotal BETWEEN 51 AND 60 THEN 'E1'
                 WHEN orgTotal BETWEEN 41 AND 50 THEN 'F1'
                 WHEN orgTotal = 0 AND ABcheck <> 1 THEN '0'
                 ELSE 'AB'
               END
FROM   MARKS_ENTRY
       CROSS apply (SELECT COALESCE(ASG_MARK, 0)+COALESCE(TH_MARKS, 0)+COALESCE(PR_MARCHAR, 0),
                           CASE
                             WHEN ASG_MARK IS NULL AND TH_MARKS IS NULL AND PR_MARCHAR IS NULL THEN 1 ELSE 0
                           END) tc (orgTotal, ABcheck)

View schema
Execution time: 0,03 sec, rows selected: 6, rows affected: 11, absolute service time: 0,19 sec