MySQL 8.0

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create table test (a int, b int)
alter table test add c int generated always as (a-b) stored
insert into test (a, b) values (1, 5), (5, 2), (4, 4), (6, 4)
select * from test


a
b
c


1
5
-4

5
2
3

4
4
0

6
4
2