Oracle 18c

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select * from V$VERSION;


BANNER
BANNER_FULL
BANNER_LEGACY
CON_ID


Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
Version 18.4.0.0.0
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production
0


create table phone (
    client_id number(6) not null,
    active number(1) not null check (active IN (0, 1)),
    value varchar2(15))
insert all
    into phone (client_id, active, value) values (10, 0, '1111')
    into phone (client_id, active, value) values (10, 1, '3333')
    into phone (client_id, active, value) values (15, 0, '5555')
    into phone (client_id, active, value) values (15, 1, '6666')
    into phone (client_id, active, value) values (15, 0, '7777')
select 1 from dual    
5 rows affected

create unique index ix1 on phone (case active when 1 then client_id end)
select * from phone


CLIENT_ID
ACTIVE
VALUE


10
0
1111

10
1
3333

15
0
5555

15
1
6666

15
0
7777