MySQL 8.0

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CREATE TABLE t1( id INT, column1 VARCHAR(50), column2 INT, column3 INT );
INSERT INTO t1 VALUES(1,'north',10,20);
INSERT INTO t1 VALUES(2,'south',30,40);
ALTER TABLE t1 ADD COLUMN
(
Destination VARCHAR(15) 
GENERATED ALWAYS AS 
    (
    CASE WHEN column1 = "north" THEN
           column2
         WHEN column1 = "south" THEN   
           column3
    END 
    )
STORED
)

Records: 2  Duplicates: 0  Warnings: 0


SELECT *
  FROM t1
  


id
column1
column2
column3
Destination


1
north
10
20
10

2
south
30
40
40