Constraint based on existing values for a column in the table

Question

I have a table as follows
create table MS_FAV_ACCT
NICKNAME VARCHAR2(50) not null,
ACCOUNT VARCHAR2(6) not null,
SUB_ACCOUNT VARCHAR2(3) not null,
DETAIL VARCHAR2(4) not null,
ICID VARCHAR2(3) not null,
SEGMENT VARCHAR2(2) not null,
PRIMARY_ACCT VARCHAR2(1) not null
I want to have a constraint such that there can be only one row with PRIMARY_ACCT ='Y' there could be multiple rows with value 'N'.
I have put a CHECK constraint on this column to check for values Y or N but I want to be able to check this condition too that only one row can have PRIMARY_ACCT as Y .
I saw a thread on this forum regarding using UNIQUE INDEX with case when but didnt understand how I could use it in my case.
Could anyone please help?

Answer 1

You only want to have 1 row in the table with an identifier of 'Y'?
That is what i understood, in which case you can use something like this.
ME_XE?create table MS_FAV_ACCT
  2  (
  3     NICKNAME VARCHAR2(50) not null,
  4     ACCOUNT VARCHAR2(6) not null,
  5     SUB_ACCOUNT VARCHAR2(3) not null,
  6     DETAIL VARCHAR2(4) not null,
  7     ICID VARCHAR2(3) not null,
  8     SEGMENT VARCHAR2(2) not null,
  9     PRIMARY_ACCT VARCHAR2(1) not null
10  );
Table created.
Elapsed: 00:00:00.03
ME_XE?CREATE UNIQUE INDEX MS_FAV_ACCT_U01 ON MS_FAV_ACCT (CASE WHEN  PRIMARY_ACCT = 'Y' THEN 1 ELSE NULL END);
Index created.
Elapsed: 00:00:00.01
ME_XE?
ME_XE?INSERT INTO MS_FAV_ACCT VALUES('ONE','TWO', 'U','I','X','A','N');
1 row created.
Elapsed: 00:00:00.00
ME_XE?INSERT INTO MS_FAV_ACCT VALUES('ONE','TWO', 'U','I','X','A','Y');
1 row created.
Elapsed: 00:00:00.00
ME_XE?INSERT INTO MS_FAV_ACCT VALUES('ONE','TWO', 'U','I','X','A','N');
1 row created.
Elapsed: 00:00:00.01
ME_XE?INSERT INTO MS_FAV_ACCT VALUES('ONE','TWO', 'U','I','X','A','Y');
INSERT INTO MS_FAV_ACCT VALUES('ONE','TWO', 'U','I','X','A','Y')
ERROR at line 1:
ORA-00001: unique constraint (TFORSYTH.MS_FAV_ACCT_U01) violated