SQL Server 2022

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CREATE TABLE your_table (
  column_A INT,
  column_B VARCHAR(255)
);
INSERT INTO your_table (column_A, column_B)
VALUES 
  (1, 'abc'),
  (0, 'abc'),
  (1, 'abc');
3 rows affected

UPDATE your_table
SET column_B = CASE WHEN column_A = 1 THEN column_B ELSE '' END;
3 rows affected

select * from your_table


column_A
column_B


1
abc

0


1
abc