i have table A and table B. I have a bridge table called tableC
in table C i have:
ID
tableA_ID
tableB_ID
ID is the primary key.
i also want to enforce the combination of tableA_ID and tableB_ID to be unique so there are no duplicate records.
how do i enforce this?
create unique index myIdx on tableC(tableA_ID, tableB_ID)
or whatever the syntax for your particular database system is.
Make the PRIMARY KEY tableA_ID and tableB_ID, EXCLUDING ID
lets say we have a table TABLEA with values
tableAID
1
2
3
and table TABLEB with values
tableBID
4
5
6
making the primary key (ID, tableA_ID, tableB_ID) will not work eg.
ID | tableAID | tableBID
1 | 1 | 4
2 | 1 | 4
will work fine with the above pk, but you need PRIMARY KEY (tableA_ID, tableB_ID)
Drop the ID column then make the other two columns the primary key and their uniqueness will be enforced by the database server.
It's not really necessary to have the ID column - even though it serves as a handy way of referencing a particular record - as the uniqueness of the other two columns will mean that they are sufficient to reference a particular record.
You may also want to put an index on this table, that includes bothe columns, to make access faster.
Related
Let's say I have two tables and I'm doing all the operations in .NET Core 2 Web API.
Table A:
Id,
SomeValue,
TeamName
Table B:
Id,
Fk_Id_a (references Id in table A),
OtherValue,
TeamName
I can add and get records from table B indepedently.
But for every record in Table B TeamName has to be the same as for it's corresponidng Fk_Id_a in Table A.
Assume these values comes in:
{
"Fk_Id_a": 3,
"SomeValue": "test val",
"TeamName": "Super team"
}
Which way would be better to check it in terms of performance? 1ST way requires two connections, when 2nd requires storing some extra keys etc.
1ST WAY:
get record from Table A for Fk_Id_a (3),
check if TeamName is the same as in coming request (Super team),
do the rest of the logic
2ND WAY:
using compound foreign keys and indexes:
TableA has alternate unique key (Id, TeamName)
TableB has foreign compound key (Fk_Id_a, TeamName) that references TableA (Id, TeamName)
SQL SCRIPT TO SHOW:
ALTER TABLE Observation
ADD UNIQUE (Id, PowelTeamId)
GO
ALTER TABLE ObservationPicturesId
ADD FOREIGN KEY(ObservationId, PowelTeamId)
REFERENCES Observation(Id, PowelTeamId)
ON DELETE CASCADE
ON UPDATE CASCADE
EDIT: Simple example how the tables might look like. TeamName has to be valid for FK referenced value in Table A.
Table A
ID | ObservationTitle | TeamName
---------------------------------------
1 | Fire damage | CX_team
2 | Water damage | CX_team
3 | Wind damage | Dd_WP3
Table B
ID | PictureId | AddedBy | TeamName | TableA_ID_FK
-----------------------------------------------------
1 | Fire | James | CX_team | 1
2 | Water | Andrew | CX_team | 1
3 | Wind | John | Dd_WP3 | 3
Performance wise, the 2nd option would be faster because there is no comparison to check (the foreign key will force that they match when inserting, updating or deleting) when selecting the rows from the table. It would also make a unique index on table A.
That being said, there is something very fishy about the structure you mention. First of all why is the TeamName repeated in table B? If a row in table B is "valid" only when the TeamName match, then you should enforce that no row should be inserted with a different TeamName, throught the ID foreign key (and not actually storing the TeamName value). If there are records on table B that represent another thing rather than the entity that is linked to table A then you should split it onto another table or just update the foreign key column when the team matches and not always.
The issue is that you are using a foreign key as a partial link, making the relationship valid only when an additional condition is true.
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 two tables, data/model is fake for simplicity purposes:
Table A:
Order ID Delivered
1 Y
2 N
3 Y
And
Table B:
Order ID Customer ID
1 123
1 234
1 455
2 789
Order ID is a primary key on Table A, and I want to use it as a Foreign Key on Table B.
Is this acceptable, given that Order ID on Table B is not unique?
Please ignore any normalisation/structural issues, my question is simply whether you can have a non-unique foreign key, I just thought the illustration would help..
Thanks,
Dearg
Is this acceptable, given that Order ID on Table B is not unique?
Yes, absolutely. This is the standard way of modeling a 1:many relationship
You should nevertheless find a primary key for TableB. If a customer cannot be assigned to more than one order, then using (order_id, customer_id) as the PK would make sense.
I have a table which has these columns:
Id (Primary Key): the id.
OwnerId (Foreign Key): the id of the owner, which resides in another table.
TypeId (Foreign Key): the type of thing this record represents. There are a finite number of types, which are represented in another table. This links to that table.
TypeCreatorId (ForeignKey): the owner of the type represented by TypeId.
SourceId (Foreign Key): this isn't important to this question.
I need to constrain this table such that for each Id, there can be only one of each TypeCreatorId. I hope that makes sense!
For SQL Server, you have two options:
create a UNIQUE CONSTRAINT
ALTER TABLE dbo.YourTable
ADD CONSTRAINT UNIQ_Id_TypeCreator UNIQUE(Id, TypeCreatorId)
create a UNIQUE INDEX:
CREATE UNIQUE INDEX UIX_YourTable_ID_TypeCreator
ON dbo.YourTable(Id, TypeCreatorId)
Basically, both things achieve the same thing - you cannot have two rows with the same (Id, TypeCreatorId) values.
Simply create a unique index on OwnerId and TypeCreatorId.
An example using MySQL (sorry, I don't use SQL Server):
alter table yourTable
add unique index idx_newIndex(OwnerId, TypeCreatorId);
Example. I'll just put here what would happen with this new unique index:
OwnerId | TypeCreatorId
--------+--------------
1 | 1
1 | 2 -- This is Ok
2 | 1 -- Ok too
2 | 2 -- Ok again
1 | 2 -- THIS WON'T BE ALLOWED because it would be a duplicate
In a table I've got 3 columns:
id
tag1
tag2
id is a primary key.
And i only want one unique tag1-tag2-combination in that table.
eg if one entry looks like:
id: 1
tag1: cat
tag2: dog
I dont want a second entry like this one beneath to get inserted:
id: 2
tag1: cat
tag2: dog
So i made all 3 columns primary keys but the problem is that then the second entry would get inserted since it looks in the combination of all 3 of them.
How do i solve this so that only the combination of the tag1 and tag2 is unique?
UPDATE: I added a unique contraint on tag1 and tag2. however, its still possible to insert:
id: 3
tag1: dog
tag2: cat
Is there a way to prevent this?
You should leave ID as the primary key, and then can create a unique constraint for the tag1 and tag2:
ALTER TABLE my_table ADD CONSTRAINT uc_tags UNIQUE (tag1, tag2)
With the unique constraint, you will be guaranteed that you will never have two rows with duplicate tag1 and tag2 values.
EDIT:
Further to your last update, you cannot enforce that with unique constraints. Keep in mind that for the database a record with (tag1 = dog, tag2 = cat) is totally different from a record with (tag1 = cat, tag2 = dog).
Probably your best bet is to redesign your database schema, as follows:
Table "tags"
Table "messages" (or whatever you are tagging)
Table "tags_messages" with the following fields (message_id, tag_id)
Then you can simply set (message_id, tag_id) of the "tag_messages" table as a primary key. This will automatically enforce that there cannot be any message with a duplicate tag.
Some sample data:
Table: messages
message_id | title
-------------+------------------
1 | some message
2 | another message
Table: tags
tag_id | tags
-------------+-------------------
1 | cat
2 | dog
3 | duck
4 | horse
Table: messages_tags
message_id | tag_id
-------------+-------------------
1 | 1
1 | 2
2 | 3
2 | 4
2 | 1
You can keep the primary key on the "id" column and add a unique constraint on the "tag1" and "tag2" columns. See this link.
Add a unique index that combines tag1 and tag2.
http://dev.mysql.com/doc/refman/5.1/en/create-index.html
Depending on if and when you need to use the "unique record" in other tables, it can be argued that your "id" field is unnecessary. (ID here is a surrogate key) If you won't be using the "id" field in another table, then is really makes more sense to make your primary key the (tag1, tag2) and to remove the "id" column all together.
I guess the question is, Why would you do it this way? It would help to know the business reason.
You can always SELECT DISTINCT to only get the rows with unique values.
If you have some control over the order of insertion and update you can enforce uniqueness of permutation:
alter table t23
add constraint tags_ck check (tag1 < tag2)
/
alter table t23
add constraint tags_uk unique (tag1, tag2)
/
This works because the check constraint rejects ('dog','cat') as an invalid combination. Consequently the unique constraint can ensure that there is only evy one record with that particular permutation of tags.
As a solution this does require some intervention at insert and update time, which may be enough to sink this implementation for you. I know of an elegant solution whcih woks in Oracle, using a function-based index (I posted it here) but I don't think MySQL supports a similar type of index.