How to create this check constraint

Question

create table emplt
( emplt_pk number,
indvl_pk number,
start_dt date,
end_dt date,
lct_fl char(1),
sup_fl char(1),
br_pk number,
nro_pk number,
default_fl char(1) default 'N' );
INSERT INTO emplt
values(
1001, 101, to_date('01-01-2005', 'MM-DD-YYYY' ), NULL, 'Y','N',123,NULL,NULL );
INSERT INTO emplt
values(
1002, 101, to_date('02-01-2005', 'MM-DD-YYYY' ), NULL, 'Y','N',NULL,0001,NULL );
INSERT INTO emplt
values(
1003, 102, to_date('02-01-2005', 'MM-DD-YYYY' ), NULL, 'Y','N',NULL,0001,NULL );
Is it possible to create a check constraint on this table that enforces that for a given indvl_pk, br_pk and nro_pk
there is only one row with dflt_fl = 'Y'?
Thanks in advance!

Answer 1

Say,
SQL> create unique index empli_ui on emplt(case default_fl when 'Y' then indvl_pk end,
  2  case default_fl when 'Y' then br_pk end,
  3  case default_fl when 'Y' then nro_pk end
  4  )
  5  /
&nbsp
Index created.
&nbsp
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(1,2,3,'J');
&nbsp
1 row created.
&nbsp
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(1,2,3,'J');
&nbsp
1 row created.
&nbsp
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(3,2,1,'A');
&nbsp
1 row created.
&nbsp
SQL>
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(3,2,1,'A');
&nbsp
1 row created.
&nbsp
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(3,2,1,'Y');
&nbsp
1 row created.
&nbsp
SQL>
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(3,2,1,'Y');
insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.EMPLI_UI) violated
&nbsp
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(1,2,3,'Y');
&nbsp
1 row created.
&nbsp
SQL> insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
  2  values(1,2,3,'Y');
insert into emplt (indvl_pk,br_pk,nro_pk,default_fl)
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.EMPLI_UI) violatedRgds.