I have a problem like
Table A:
-- TableBCId
Table B:
-- Id
Table C:
-- Id
I am looking for a way to create a foreign key table A where an entry can be either in table B or table C
Example entries:
Table A:
-- TableBCId: 1
-- TableBCId: 2
Table B:
-- Id: 1
Table C:
-- Id: 2
I want to avoid if possible:
- Two columns in table A
- Default values
- Additional tables
- Creation of an base entity is not possible
Every idea welcome
The normal way to implement this requirement is with 2 columns, 2 foreign key constraints, and a check constraint to ensure exactly of of the columns populated (if this is a requirement):
create table a
( ...
, b_id references b
, c_id references c
, constraint <name> check ( (b_id is null and c_id is not null)
or (b_id is not null and c_id is null)
)
);
You could, if it helps your UI, create a view over that table that combines B_ID and C_ID into a single column.
But you have said you don't want 2 columns, why is that?
The reason why this is hard is because the data model is wrong. A foreign key references only one table. A table can have more than one foreign key but each is separate. Apart from anything else, how would you know whether bc_id referenced b.id or c.id?
One explanation for this scenario is that table A should really be two tables, BA referencing B and CA referencing C. Alternatively A should reference a super-type table, of which B and C are sub-types. Without knowing the actual business domain it's hard to be sure.
Anyway, the path of least change is two columns.
You can use a insert/update trigger on your Table_A.
Maybe something like this:
CREATE TRIGGER Table_a_trgr
BEFORE INSERT OR UPDATE
on Table_a
FOR EACH ROW
DECLARE
c_row NUMBER;
BEGIN
SELECT count(*)
INTO c_row
FROM (
SELECT ID FROM table_b WHERE id = :NEW.TableBCId
UNION ALL
SELECT ID FROM table_c WHERE İd = :NEW.TableBCId
)
;
IF c_row < 2 THEN
raise_application_error(-20000, 'Error, Foreign Key');
END IF;
END;
/
If you don't want to make a new column in table A.
Then you can make a parent table to B and C.
tableBC:
id
And then create a 1-1 relation with tables B and C to table BC.
tableB:
id,
parent -- 1-1 foreign key to tableBC
tableC:
id,
parent -- 1-1 foreign key to tableBC
Now, in table A
tableA:
id,
TableBCId -- foreign key to tableBC
We have solved a similar problem using this approach.
Related
We have realized that we need to use Oracle (12 c) interval partitioning.We have an hierarchical entity model with lots of #OneToMany relationships.We want to use "partitioning by range" (day,month... ) on the "parent/root" entity (A) and "partition by reference" on all child entities (B).
From the Oracle documentation: "Reference partitioning allows the partitioning of two tables related to one another by referential constraints. The partitioning key is resolved through an existing parent-child relationship, enforced by enabled and active primary key and foreign key constraints." The problem is that child entities (B) can refer to other entities ( C) that they don't have any link with the "parent/root" entity (A). I can create partitions on A and on B but when I want to drop the partition on A ( partition on B on cascade), I get an error integrity error and it fails. it works only if I delete all records on C and B and then partition them. I don't want to do that as it's not efficient and slow compared to dropping partitions directly
Please is there a way to create a partition on table C based on A(creation_date) without adding any foreign constraint between A and C?
Small example to illustrate the case
A - parent entity
B - child entity to A
C - child entity to B
create table
A (
id number primary key,
creation_date date
)
partition by range (creation_date)
INTERVAL(NUMTOYMINTERVAL(1, 'MONTH'))
(
partition p1 values less than (to_date('20180501','yyyymmdd'))
);
create table
B (
id number primary key,
value varchar2(5),
a_id number not null,
constraint fk_ba foreign key (a_id) references A
)
partition by reference(fk_ba);
create table
C (
id number primary key,
code varchar2(5),
b_id number not null,
constraint fk_cb foreign key (b_id) references B
);
It was not clear in the documentation that partition by reference is transitive ( A->B->C). I have discovered it by testing. we can create table C "partition by reference (fk_cb)" with the result that when we drop A partition the matching B and C partitions get dropped.
I've been struggling for some time now to figure out a way to create a composite foreign key in liquibase.
I have a table A which has a composite PK, let's say (id1, id2). I'm trying to make another table B, in which the A.PK is mapped as a FK.
I'm using liquibase with YAML and something doesn't seem to add up.
I've tried adding the FK when creating the table (so in the column tag)
- column:
name: id1_id2
type: int
constraints:
nullable: false
foreignKeyName: fk_id1_id2
references: A(id1, id2)
Unfortunately this syntax returns an error:
Caused by: java.sql.SQLSyntaxErrorException: ORA-02256: number of referencing columns must match referenced columns
Another thing that I've tried is creating the table first, with the column for the desired FK and try to add a FK constraint on that column. This doesn't throw any error but it does nothing (also the log for LB says "empty" in the description)
changes:
- addForeignKeyContraint:
baseColumnNames: id1, id2
baseTableName: B
constraintName: fb_id1_id2
referencedColumnNames: id1, id2
referencedTableName: A
Any help would be much appreciated.
Thanks
Have you tried addForeignKeyConstraint? Something like this:
- changeSet:
id: 1
author: you
changes:
- addForeignKeyConstraint:
baseColumnNames: id1, id2
baseTableName: tableB
constraintName: FK_tableB_tableA
referencedColumnNames: id1, id2
referencedTableName: tableA
I don't use Liquibase, but here's how it is supposed to look like, as far as Oracle is concerned: if you want to create a composite foreign key (in the detail table), then it has to reference a composite primary key (in the master table).
Have a look at this example:
SQL> create table master
2 (id_1 number,
3 id_2 number,
4 constraint pk_mas primary key (id_1, id_2));
Table created.
SQL> create table detail
2 (id_det number constraint pk_det primary key,
3 --
4 id_1 number,
5 id_2 number,
6 constraint fk_det_mas foreign key (id_1, id_2) references master (id_1, id_2));
Table created.
SQL>
It just wouldn't work otherwise; that's why you got the error
ORA-02256: number of referencing columns must match referenced columns
because your detail table contained a single column (id1_id2) and tried to reference two columns in table A (id1, id2).
Brief model overview:
I have a student and a course tables. As it's many to many relation there is also a junction table student_course (id_student, id_course), with unique constraint on both columns (composite).
The problem I want to solve:
On account of a mistake, there is no a unique constraint on the code column of the course table. It should as code column should uniquely identify a course. As a result there are two rows in the course table with the same value in the code column. I want to remove that duplicate, check that there is no other duplicates and add a unique constraint on the code column. Without loosing relations with student table.
My approach to solve the issue:
I have create a procedure that should do what I want.
CREATE OR REPLACE PROCEDURE REMOVE_COURSES
(
v_course_code IN VARCHAR2,
v_course_price IN VARCHAR2
)
AS
new_course_id NUMBER;
BEGIN
INSERT INTO course (CODE, PRICE) VALUES (v_course_code, v_course_price)
RETURNING ID INTO new_course_id;
FOR c_course_to_overwrite IN (SELECT *
FROM course
WHERE code = v_course_code AND id != new_course_id) LOOP
UPDATE student_course SET id_course = new_course_id WHERE id_course = c_course_to_overwrite.id;
DELETE FROM course WHERE id = c_course_to_overwrite.id;
END LOOP;
END REMOVE_COURSES;
/
Main problem I want to solve:
The procedure keeps giving me an error about unique constraint violation on student_course table. But I am really not sure how it's possible as I am using new_course_id, so there is no chance that in the junction table there are two rows with the same id_student, id_course. What do I need to fix ?
Miscellaneous:
I want to solve that issue using procedure only for learning purposes
EDITED:
CREATE TABLE student (
id NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY,
name VARCHAR2(150) NOT NULL,
PRIMARY KEY (id)
);
ALTER TABLE student MODIFY ID
GENERATED BY DEFAULT ON NULL AS IDENTITY (START WITH LIMIT VALUE);
CREATE TABLE course (
id NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY,
code VARCHAR2(255) NOT NULL,
PRIMARY KEY (id)
);
ALTER TABLE course MODIFY ID
GENERATED BY DEFAULT ON NULL AS IDENTITY (START WITH LIMIT VALUE);
CREATE TABLE student_course (
id_student NUMBER NOT NULL,
id_course NUMBER NOT NULL,
PRIMARY KEY (id_student, id_course),
CONSTRAINT student_fk FOREIGN KEY (id_student) REFERENCES student (id),
CONSTRAINT course_fk FOREIGN KEY (id_course) REFERENCES course (id)
);
insert into student (name) values ('John');
INSERT INTO course (ID, CODE) VALUES (1, 'C_13');
INSERT INTO course (ID, CODE) VALUES (2, 'C_13');
commit;
INSERT INTO STUDENT_COURSE (ID_STUDENT, ID_COURSE) VALUES (1, 1);
INSERT INTO STUDENT_COURSE (ID_STUDENT, ID_COURSE) VALUES (1, 2);
commit;
CALL REMOVE_COURSES('C_13');
[23000][1] ORA-00001: unique constraint (SYS_C0014983) violated ORA-06512: near "REMOVE_COURSES", line 8
Rather than removing one of the duplicate codes, you're creating a third course with the same code, and trying to move all students on either of the old courses onto the new one. The error suggests you have students who are already enrolled on both of the old courses.
Your cursor loop query is:
SELECT *
FROM course
WHERE code = v_course_code AND id != new_course_id
That will find all junction records for both old versions of the code, and the update then sets all of those junction records to the same new ID.
If there are any students listed against both old IDs for the code - which would be allowed by your composite unique key - then they will both be updated to the same new ID.
So say the courses you're looking at are [updated for your example code]:
ID CODE
-- ----
1 C_13
2 C_13
and you have junction records for a student for both courses, like:
ID_STUDENT ID_COURSE
---------- ---------
1 1
1 2
You are creating a new course:
ID CODE
-- ----
3 C_13
Your cursor loop looks for code = 'ABC' and ID != 3, which finds IDs 1 and 2. So in the first iteration of the loop up update the rows with ID 1, so now you have:
ID_STUDENT ID_COURSE
---------- ---------
1 3
1 2
Then in the second iteration you try to update the rows with ID 2, which would attempt to produce:
ID_STUDENT ID_COURSE
---------- ---------
1 3
1 3
which would break the unique constraint - hence the error.
You probably don't want to create a new course at all, but either way, you need to remove duplicate records from student_course - that is, rows which will become duplicates when updated. Basically you need to find students with entries for both existing course IDs, and delete either of them. If you don't care which this would do it:
delete from student_course sc1
where id_course in (
select id
from course
where code = 'C_13'
)
and exists (
select null
from student_course sc2
join course c on c.id = sc.id_course
where sc2.id_student = sc1.id_student
and sc2.id_course > sc1.id_course
and c.code = 'C_13'
);
but there are other (probably better) ways.
You then have the choice of updating all remaining junction records for both old IDs to your new ID; or to consolidate on one of the old IDs and remove the other.
(Your question implies you want to solve the overall task yourself, so I'll refrain from trying to provide a complete solution - this just hopefully helps you understand and resolve your main problem...)
I have three tables:
A {op_id, op_name, .}
B {b_id, op_id, supplier_id, .}
C {c_id, op_id, op_id2, supplier_id, relation, .}
Table B and C have op_id which is foreign key from table A. In table A op_id is primary key, in table B b_id is primary key and in table C c_id is primary key. In table B supplier_id and op_id may have duplicate records. Now I want to add constraint so that if I delete records from Table B for op_id and if a relationship record exists for op_id in table C then it should not allow me to delete. Is it possible through constraint?
Add the constraints normally as specified by you like below
Table A op_id is primary key,
in table B b_id is primary key
and in table C c_id is primary key
Table B and C have op_id which is foreign key from table A
For the last condition that you have specified write a trigger before delete on table B and check if the op_id is present in the table C or not - if yes throw an error else allow it.
Hope this helps.
Most of your key constraints are fairly trivial, the difficult bit is the last part, ensuring that you cannot remove rows from B without first removing them from C.
One approach that might work here would be to create a fast-refreshable "on commit" materialised view which returns a count of the number of unique OP_IDs that exist in C but that are not in B. You could then add a check constraint to the view which enforces that the count must always be zero. This constraint would be enforced at the point of commit and would prevent you deleting from B without first deleting from C (deleting from B whilst matching records exist in C would violate the check constraint as the count in the materialized view would be greater than 0). A side effect of this would be that you wouldn't be able to insert into C without first inserting into B.
Table 1:
Name, x1-X2, fk1, fk2.
Table 2:
K1(parent for table 1),X
How to update table 1 whose second column x1-x2 depands on fk1,fk2 from Table 2
Table1:
a,1.0,1,2
b,-3.0,2,3
Table 2
1,4.0
2,5.0
3,2.0
With this setup:
CREATE TABLE table2 (k NUMBER PRIMARY KEY, x NUMBER);
CREATE TABLE table1 (
NAME VARCHAR2(10) PRIMARY KEY,
diff NUMBER,
fk1 NUMBER REFERENCES table2,
fk2 NUMBER REFERENCES table2);
the following update will "refresh" the colum table1.diff with the values of table2:
SQL> UPDATE (SELECT child.diff old_diff, parent2.x - parent1.x new_diff
2 FROM table1 child
3 JOIN table2 parent1 ON (child.fk1 = parent1.k)
4 JOIN table2 parent2 ON (child.fk2 = parent2.k))
5 SET old_diff = new_diff
6 WHERE old_diff != new_diff
7 OR (old_diff IS NULL AND new_diff IS NOT NULL)
8 OR (old_diff IS NOT NULL AND new_diff IS NULL);
Only the rows that need updating will be refreshed (thanks to the where clause).
Not quite sure I understand the question, referential integrity constraints do not prevent you from updating a table. If you need to do a lot of work in a transaction you can look into Deferred Constraints. Is there any chance you could clarify what you mean?
Not sure exactly what the problem is, maybe you need to rephrase the question a little.
In general, the foreign key constraint makes sure that there exists a corresponding row in the referenced table.
When you update a row with a foreign key, and try to set a value that does not point to such a master row, you will get an error, either immediately or when you commit (depending on when the constraint is enforced, it can be deferred).
Other than that, the update is no different than any other update.
Since the data in table 1 is dependent on the data in table 2 you won't be able to "Update" table 1 directly.
You will have to perform the update on table 2 and then recalculate table 1.
You could do it inside a transaction or perhaps a trigger on table 2.
Another option may be to have table one only hold the foreign keys and the name and then create a view that calculates the X1-X2 value.
EDIT
After looking at the sample data, I don't think you can unambiguously have table 2 be updates as a result of an update on table 1.
For example if you update the second column of table 1 to be 43, how would you know what values to set the specific rows of table 2 (it could be 40 and 3, 20 and 23, etc.)