ORA-02291: integrity constraint (string.string) violated - parent key not found - oracle

Question

I am created 2 tables named entry and team based on the attached logical model.
I didn't have any problems with the create table as I created the tables first. altered the table to have the primary keys. and by the end add foreign keys.
CREATE TABLE entry (
event_id NUMBER(6) NOT NULL,
entry_no NUMBER(5) NOT NULL,
entry_starttime DATE,
entry_finishtime DATE,
comp_no NUMBER(5) NOT NULL,
team_id NUMBER(3),
char_id NUMBER(3)
);
ALTER TABLE entry ADD CONSTRAINT entry_pk PRIMARY KEY ( event_id,
entry_no );
CREATE TABLE team (
team_id NUMBER(3) NOT NULL,
team_name VARCHAR(30) NOT NULL,
carn_date DATE NOT NULL,
team_no_members NUMBER(2) NOT NULL,
event_id NUMBER(6) NOT NULL,
entry_no NUMBER(5) NOT NULL,
char_id NUMBER(3)
);
ALTER TABLE team ADD CONSTRAINT team_pk PRIMARY KEY ( team_id );
ALTER TABLE team ADD CONSTRAINT team_nk UNIQUE ( team_name,
carn_date );
ALTER TABLE entry
ADD CONSTRAINT team_entry FOREIGN KEY ( team_id )
REFERENCES team ( team_id );
ALTER TABLE team
ADD CONSTRAINT entry_team FOREIGN KEY ( event_id,
entry_no )
REFERENCES entry ( event_id,
entry_no );
My problems come in when i start inserting values to the tables.
team_no is a foreign key in entry table (but could be null). event_id and entry_no are foreign keys (not null) in team table.
INSERT INTO entry (
event_id,
entry_no,
entry_starttime,
entry_finishtime,
comp_no,
team_id,
char_id
) VALUES (
6,
2,
TO_DATE('08:30', 'HH:MI'),
TO_DATE('08:50', 'HH:MI'),
10,
NULL,
1
);
INSERT INTO team (
team_id,
team_name,
carn_date,
team_no_members,
event_id,
entry_no,
char_id
) VALUES (
5,
'Turner Hall',
TO_DATE('24/SEP/2021', 'DD/MON/YYYY'),
2,
2,
1,
4
);
it gives me a ORA-02291: integrity constraint (string.string) violated - parent key not found error.
How can I fix this?
Thanks!

Answer 1

Fix your design first.
Start out with some simple sentences that define the what each thing is (Carnival, Event, Entry, Team, Competitor, Charity).
Once you know what they mean then you can write some more simple sentences defining the relationships.
Only once you have it straight in your head what everything is and how it is related then you can create the entity-relationship diagram and that diagram should be a visual representation that has a 1:1 correspondence to your simple sentence descriptions.
If you find you want to add a constraint in the ER diagram that does not have a description then write the simple sentence description first and think long and hard about whether what you have written makes logical sense; if it does not then do not add the relationship.
So you can start with:
There are people (Competitors).
The Competitors are grouped into multiple Teams.
Each Team can have many members (Competitors) but exactly one of those is the team leader.
There are Carnivals.
Each Carnival can host multiple competitive Events.
An Event requires an Entry from each Team.
Each Team can have multiple Entrys into different Events.
Each Team can only have one Entry into each Event.
Each Event can have multiple Entry from different teams.
Each Team supports exactly one Charity.
From that description, there is no mention of competitors and events in the same relationship so, if that was the case, then in your ER diagram there should not be any direct relationship between competitors and events; instead there are relationships from each to a team.
Similarly, the team leader relationship is from a Team to a Competitor and not to an Entry.
You may have different ideas of what the tables and relationships mean but you will find that by setting everything out in simple sentences it will force you to understand what everything means and making sure it makes sense.
Once you fix your design then you will find that you have eliminated the problem of cyclic references that prevent you from inserting data without disabling constraints.