MySQL 8.0

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

create  table foo (custom_field_id int , val int);
create unique index idx1 on foo(val, (if(custom_field_id = 7, custom_field_id,  null)));
insert into foo values (7,1)
insert into foo values (7,1)
Duplicate entry '1-7' for key 'foo.idx1'

insert into foo values (7,2)
insert into foo values (6,1)
insert into foo values (6,1)
select *, if(custom_field_id = 7, custom_field_id,  null) from  foo;


custom_field_id
val
if(custom_field_id = 7, custom_field_id,  null)


7
1
7

7
2
7

6
1
null

6
1
null