Can this business logic be enforced by a conditional database constraint?

I am trying to duplicate the business logic embodied an intranet C# web application in the database so that other databases can access it and work under the same rules. This "rule" seems difficult to implement without using hacks.

CREATE TABLE CASE_STAGE
(
  ID                        NUMBER(9)           PRIMARY KEY NOT NULL, 
  STAGE_ID                  NUMBER(9)           NOT NULL,
  CASE_PHASE_ID             NUMBER(9)           NOT NULL,
  DATE_CREATED              TIMESTAMP(6)        DEFAULT CURRENT_TIMESTAMP     NOT NULL,
  END_REASON_ID             NUMBER(9),
  PREVIOUS_CASE_STAGE_ID    NUMBER(9),
  "CURRENT"                 NUMBER(1)           NOT NULL,
  DATE_CLOSED               TIMESTAMP(6)        DEFAULT NULL
);

and

CREATE TABLE CASE_RECOMMENDATION
(
  CASE_ID                   NUMBER(9)           NOT NULL,
  RECOMMENDATION_ID         NUMBER(9)           NOT NULL,
  "ORDER"                   NUMBER(9)           NOT NULL,
  DATE_CREATED              TIMESTAMP(6)        DEFAULT CURRENT_TIMESTAMP     NOT NULL,
  CASE_STAGE_ID             NUMBER(9)           NOT NULL
);

ALTER TABLE CASE_RECOMMENDATION ADD (
  CONSTRAINT SYS_C00000
 PRIMARY KEY
 (CASE_ID, RECOMMENDATION_ID));

The business logic can be summed up as

When Inserting into CASE_STAGE
If CASE_STAGE.STAGE_ID = 1646
THEN
 CASE_STAGE.PREVIOUS_STAGE_ID must be found in CASE_RECOMMENDATION.CASE_STAGE_ID

Can this logic be embodied in a Check constraint or is an ugly trigger the only way?

Edit:

• For all values of CASE_STAGE.STAGE_ID the value for PREVIOUS_STAGE_ID must be found in CASE_STAGE.ID

• The application does not allow deletions from CASE_RECOMMENDATION once it is no longer CURRENT ( ie when the value of CASE_STAGE.CURRENT is 0 this stage is closed and can no longer be changed, when = 1 this is the stage, or row, that is active and can be changed now.)

Edit: using all the excellent ideas and comments here is a working solution to this problem

```
CREATE MATERIA

If CASE_RECOMMENDATION.CASE_STAGE_ID is unique, you can use referential integrity with a virtual column (11g+) to make it conditional:

alter table CASE_RECOMMENDATION ADD CONSTRAINT unique_case_stage unique (CASE_STAGE_ID);

-- virtual column only defined when stage_id=1646
alter table CASE_STAGE add 
   (case_1646 as (case when stage_id=1646 then previous_case_stage_id end));

-- check that the virtual column is defined when stage_id=1646
alter table case_stage add 
    constraint chk_1646 check ( stage_id!=1646 or previous_case_stage_id is not null);

-- referential integrity
alter table case_stage add 
     constraint fk_1646 foreign key (case_1646) 
     references case_recommendation (case_stage_id);

Let's check:

SQL> insert into  CASE_STAGE values (1, 1, 1, sysdate, null, null, 1, null, default);

1 row(s) inserted.

SQL> insert into CASE_RECOMMENDATION values (1, 1, 1, sysdate, 1);

1 row(s) inserted.

SQL> -- fails because previous_case_stage_id is null
SQL> insert into CASE_STAGE values (2, 1646, 1, sysdate, null, null, 1, null, default);

ORA-02290: check constraint (VNZ_TEST3.CHK_1646) violated

SQL> -- fails because previous_case_stage_id doesn't exist in CASE_RECOMMENDATION
SQL> insert into CASE_STAGE values (2, 1646, 1, sysdate, null, 2, 1, null, default); 

ORA-02291: integrity constraint (VNZ_TEST3.FK_1646) violated - parent key not found

SQL> -- succeeds !
SQL> insert into CASE_STAGE values (2, 1646, 1, sysdate, null, 1, 1, null, default); 

1 row(s) inserted.

StackExchange Database Administrators Q#24280, answer score: 6