MySQL 5.7

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create table TableA(Id int, Function varchar(10));
insert into TableA(Id, Function) values
(1, 'code1'), (2, 'code2'), (3, 'code3'), (4, 'code4');
create table TableB(Id int, Function varchar(10));
insert into TableB(Id, Function) values
(1, 'code1'), (2, 'code4'), (3, 'code5');
select a.*,
  exists (select 1 from TableB b where b.function = a.function) compare
from TableA a  


Id
Function
compare


1
code1
1

2
code2
0

3
code3
0

4
code4
1


select a.*,
  case 
    when exists (select 1 from TableB b where b.function = a.function) then 'true'
    else 'false'
  end compare
from TableA a  


Id
Function
compare


1
code1
true

2
code2
false

3
code3
false

4
code4
true