Postgres 15

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE my_table (
  a int
, b int
, c int
);

INSERT INTO my_table (a,b,c) VALUES
  (3,2, 1)
, (3,2, 0)
, (3,2,-1)
RETURNING *;

CREATE TABLE




a
b
c


3
2
1

3
2
0

3
2
-1



INSERT 0 3


-- don't store & compute on the fly:
SELECT *, CASE WHEN c<0 THEN a+b ELSE a-b END AS d
FROM   my_table;


a
b
c
d


3
2
1
1

3
2
0
1

3
2
-1
5



SELECT 3


-- you might use a view for that:
CREATE VIEW my_view AS
SELECT *, CASE WHEN c<0 THEN a+b ELSE a-b END AS d
FROM   my_table;

TABLE my_view;

CREATE VIEW




a
b
c
d


3
2
1
1

3
2
0
1

3
2
-1
5



SELECT 3


-- In case you need it stored, use a generated column:
ALTER TABLE my_table
ADD column d int GENERATED ALWAYS AS (CASE WHEN c<0 THEN a+b ELSE a-b END) STORED;

TABLE my_table;

ALTER TABLE




a
b
c
d


3
2
1
1

3
2
0
1

3
2
-1
5



SELECT 3