I have 2 tables: A and B.
A many-to-many relation units them through a join table A_B.
Now, my needs evolve: an A and a B can be related by more than 1 way.
I don't know what is the more conventional way to do that.
Must I declare a new "relation_way" table that contains the different "ways" for a A to be connected to a B and use this to compose a ternary key in A_B?
I would simply add a column to a_b that states the type of the relationship, e.g. relation_type that stores e.g. owned_by or referred_to or however you want to describe that relation (your obfuscated table and column names do not help a bit in answering this).
create table a_b
(
a_id integer not null references a,
b_id integer not null references b,
relation_type text not null
);
If you allow multiple relations but with different types between two entities, then include the relation_type in the primary key of the a_b table.
If you want to restrict the possible relation types, you should create a lookup table:
create table relation_type
(
id integer primary key,
type_name varchar(20) not null unique
);
and reference that from the link table:
create table a_b
(
a_id integer not null references a,
b_id integer not null references b,
relation_type_id integer not null references relation_type,
primary key (a_id, b_id, relation_type_id)
);
Related
I've found a million things sounding kind of similar on StackOverflow, but not my case exactly. I'll simplify as much as possible:
I have two tables as follows:
CREATE TABLE B (id uuid PRIMARY KEY);
CREATE TABLE A (id uuid PRIMARY KEY, b_id uuid REFERENCES b);
There are some NULL values in A.b_id. I am trying to create a migration that does the following:
For every row in A with no b_id, create a new row in B, and assign its id to A.b_id.
How can I accomplish this in one query?
Assuming you want a distinct entry in b for every row with a missing UUID in a:
WITH upd AS (
UPDATE a
SET b_id = gen_random_uuid()
WHERE b_id IS NULL
RETURNING b_id
)
INSERT INTO b (id)
SELECT b_id FROM upd;
db<>fiddle here
This works because it's a single command, and the FK reference is only enforced at the end of the command.
See:
SET CONSTRAINTS ALL DEFERRED not working as expected
Constraint defined DEFERRABLE INITIALLY IMMEDIATE is still DEFERRED?
How do I add constraint to guard that a primary key could only be referenced once?(It could be referenced in two tables)
Each reference should have a unique value out of the primary key.
Table A
----------------------
id
1
2
3
4
Table B
----------------------
id a_id (foreign key to table A.id)
1 2
2 3
Table C
----------------------
id a_id (foreign key to table A.id)
1 1
I want something to happen to give error when try to insert a_id = 2 into table C as its used in table B already.
You can use an INSERT, UPDATE trigger on each of the child tables to ensure that the PK of the parent table that is about to be inserted or updated does not already exist in the other child table.
What you are trying to do requires another table D, that will help unify the references to A.
Table D will contain its own primary key ( Id ), a reference to table A with a UNIQUE constraint on it (call it AId ), and a third column (called "RowType") to indicate to which of the child tables (B or C) the row corresponds. You can make this column to be of type int, and assign value "0" for B and "1" for C, for example.
Then in table B you add a foreign key to D.Id, AND another column "BRowType" as foreign key to D.RowType; then you define a constraint on this column, so it can only have the value '0' ( or whatever value you have decided to correspond to this table).
For table C your constraint will limit the values to '1'.
Or course, in order to insert a record into B or C you first need to create a record in D. But once you have a record in B that references a record in D, which in turn links to a record in A, you will no longer be able to create a record in C for the same line in A - because of the UNIQUE constraint on D.AId AND the constraint on C.BRowType.
If I understand the question correctly, it sounds like you need to add a unique constraint on the column of each table that references your primary key.
For example:
Table A
----------------------
id (primary key)
1
2
3
Table B
----------------------
id a_id (foreign key to table A.id)
1 2
2 3
Set the a_id column to be UNIQUE and that way you can ensure that the primary key from Table A is not used twice. You would do that in each table which references A.id
If you want to avoid using triggers, you could create a table X with id and a unique constraint on it.
In each transaction in which you insert a record into B or C you have to insert into X as well. Both insertions will only be possible if not yet in the other table.
I have a query regarding a design problem that I faced …
There is a table A with subtypes B and C. Table A has an attribute type which tells whether the type is B or C. The common attributes of B and C are in A .
The problem is that there are no extra attributes for B .. all attributes required for B are in A already. However , there are extra attributes for C.
Is it an acceptable solution if I make tables A and C only ??… to extract entities of B I will query through the type attribute from table A
Can you refer any material ?
I also had a another confusion where table A has subtypes B,C,D . Table Z has a column that requires a value of primary id of either B or C but NOT D.
I thought of adding the primary id column of A as a foreign key reference to Z’s column and then making a trigger to ensure that the id isn't D ...
Can anyone please comment ?
Thank you !
Many people just enforce all these rules in application code. That is, they "simply" don't insert wrong data. Of course this is very fragile and depends on writing perfect application code at all times. So we want the database to enforce the constraints instead, so that wrong data can never be inserted.
CREATE TABLE A (
id INT PRIMARY KEY,
type CHAR(1) NOT NULL,
unique key (id, type)
);
CREATE TABLE B (
id INT PRIMARY KEY,
type CHAR(1) NOT NULL DEFAULT 'B',
FOREIGN KEY (id, type) REFERENCES A(id, type)
);
If you can force B.type to always be 'B' (CHECK constraint, trigger, or reference a one-row lookup table) then it can of course reference parent rows in A where type='B'. And do something similar in tables C and D, so then each row of A can be referenced by a row from only one sub-type table.
That is, if A.type is 'B' on a given row, and C.type can only be 'C', then no row of C can reference any row where A.type is 'B'.
Now if you want table Z to reference B or C but not D, you can reference by id and type, so Z also has its own type column. You can restrict Z.type by using a lookup table:
CREATE TABLE Ztypes (
type CHAR(1) PRIMARY KEY
);
INSERT INTO Ztypes VALUES ('B'), ('C');
CREATE TABLE Z (
id INT PRIMARY KEY,
Aid INT NOT NULL,
type CHAR(1) NOT NULL,
FOREIGN KEY (Aid, type) REFERENCES A(id, type),
FOREIGN KEY (type) REFERENCES Ztypes(type)
);
You've already got the answer you were looking for. But for other who run across this, it's worth researching two techniques: Class Table Inheritance and Shared Primary Key.
These two techniques used together make it fast, simple and easy to join A's data with either B's or C's data. And in this pattern, B contains only the key, but still contains usefull informaton.
Both of these techiques have their own tags.
how do you change the relationship between two tables to a many to many to many relationship in sql. Im using oracle for the DB.
thanks
Relationships among tables are almost always, ONE-TO-MANY, or ONE-TO-ONE. There is no MANY-TWO-MANY relationship between two tables. If you want a MANY-TO-MANY you will need to create intermediate relation to hold the relationship.
For example, if you want a MANY-TO-MANY relationship between table A and B you will need to create an intermediate table C:
create table a (a_id number primary key);
create table b (b_id number primary key);
-- c will hold many-to-many relationship between a and b
create table c (
a_id number not null references a(a_id),
b_id number not null references b(b_id),
primary key(a_id, b_id)
);
This isn't a big deal, but my OCD is acting up with the following problem in the database I'm creating. I'm not used to working with databases, but the data has to be stored somewhere...
Problem
I have two tables A and B.
One of the datafields is common to both tables - segments. There's a finite number of segments, and I want to write queries that connect values from A to B through their segment values, very much asif the following table structure was used:
However, as you can see the table Segments is empty. There's nothing more I want to put into that table, rather than the ID to give other table as foreign keys. I want my tables to be as simple as possible, and therefore adding another one just seems wrong.
Note also that one of these tables (A, say) is actually master, in the sense that you should be able to put any value for segment into A, but B one should first check with A before inserting.
EDIT
I tried one of the answers below:
create table A(
id int primary key identity,
segment int not null
)
create table B(
id integer primary key identity,
segment int not null
)
--Andomar's suggestion
alter table B add constraint FK_B_SegmentID
foreign key (segment) references A(segment)
This produced the following error.
Maybe I was somehow unclear that segments is not-unique in A or B and can appear many times in both tables.
Msg 1776, Level 16, State 0, Line 11 There are no primary or candidate
keys in the referenced table 'A' that match the referencing column
list in the foreign key 'FK_B_SegmentID'. Msg 1750, Level 16, State 0,
Line 11 Could not create constraint. See previous errors.
You can create a foreign key relationship directly from B.SegmentID to A.SegmentID. There's no need for the extra table.
Update: If the SegmentIDs aren't unique in TableA, then you do need the extra table to store the segment IDs, and create foreign key relationships from both tables to this table. This however is not enough to enforce that all segment IDs in TableB also occur in TableA. You could instead use triggers.
You can ensure the segment exists in A with a foreign key:
alter table B add constraint FK_B_SegmentID
foreign key (SegmentID) references A(SegmentID)
To avoid rows in B without a segment at all, make B.SegmentID not nullable:
alter table B alter column SegmentID int not null
There is no need to create a Segments table unless you want to associate extra data with a SegmentID.
As Andomar and Mark Byers wrote, you don't have to create an extra table.
You can also CASCADE UPDATEs or DELETEs on the master. Be very carefull with ON DELETE CASCADE though!
For queries use a JOIN:
SELECT *
FROM A
JOIN B ON a.SegmentID = b.SegmentID
Edit:
You have to add a UNIQUE constraint on segment_id in the "master" table to avoid duplicates there, or else the foreign key is not possible. Like this:
ALTER TABLE A ADD CONSTRAINT UNQ_A_SegmentID UNIQUE (SegmentID);
If I've understood correctly, a given segment cannot be inserted into table B unless it has also been inserted into table A. In which case, table A should reference table Segments and table B should reference table A; it would be implicit that table B ultimately references table Segments (indirectly via table A) so an explicit reference is not required. This could be done using foreign keys (e.g. no triggers required).
Because table A has its own key I assume a given segment_ID can appear in table A more than once, therefore for B to be able to reference the segment_ID value in A then a superkey would need to be defined on the compound of A_ID and segment_ID. Here's a quick sketch:
CREATE TABLE Segments
(
segment_ID INTEGER NOT NULL UNIQUE
);
CREATE TABLE A
(
A_ID INTEGER NOT NULL UNIQUE,
segment_ID INTEGER NOT NULL
REFERENCES Segments (segment_ID),
A_data INTEGER NOT NULL,
UNIQUE (segment_ID, A_ID) -- superkey
);
CREATE TABLE B
(
B_ID INTEGER NOT NULL UNIQUE,
A_ID INTEGER NOT NULL,
segment_ID INTEGER NOT NULL,
FOREIGN KEY (segment_ID, A_ID)
REFERENCES A (segment_ID, A_ID),
B_data INTEGER NOT NULL
);