SQL Server 2017

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CREATE TABLE T(
  Val VARCHAR(45),
  Seq INT,
  Sequence INT
);

INSERT INTO T VALUES
('A', 1, 2),
('C', 2, 1),
('B', 3, 3);

3 rows affected

--Fixes--

DECLARE @T VARCHAR(3) = '1'; --Try to change it to ABC too

SELECT *
FROM T
ORDER BY CASE WHEN @T='ABC' THEN Val END, --Take varchar
         CASE WHEN @T='ABC' THEN Seq -- Take INT
              ELSE Sequence -- Take INT
         END;
/*
 IF @T = ABC
 ORDER BY Val, Seq; NOT Sequence
 
 IF @T <> ABC
 ORDER BY ONLY Sequence
*/


Val
Seq
Sequence


C
2
1

A
1
2

B
3
3


--You was trying--

DECLARE @T VARCHAR(3) = 'ABC'; --wil works only when @T <> ABC

SELECT *
FROM T
ORDER BY CASE WHEN @T='ABC' THEN Seq END, --Take INT
         CASE WHEN @T='ABC' THEN Val --Take Varchar
              ELSE Sequence -- Take INT
         END;
Msg 245 Level 16 State 1 Line 5
Conversion failed when converting the varchar value 'A' to data type int.