SQL Server – Composite Foreign Key Not Verifying All Columns

foreign keysql server

Table1 (Num [PK NOT NULL], Dat [PK NOT NULL], …)

Table2 (ID [PK], Num, Dat, …)

I added this FK:

ALTER TABLE [dbo].[Table2]  WITH CHECK ADD FOREIGN KEY([Num], [Dat])
REFERENCES [dbo].[Table1] ([Num], [Dat])

But when I try to insert into Table2 the values [ Num = 1, Dat = NULL ], SQL don't give me an error and the values are inserted.

How I force SQL Server to check all columns of the foreign key?

Best Answer

A constraint of FOREIGN KEY will never forbid you from entering NULL values in nullable columns. The constraint is simply ignored for such rows.

You can make the columns not nullable, if you want to enforce this. Based on the comments though, you probably just need to add a CHECK constraint on the table, so the two columns are either null or not null, together:

ALTER TABLE dbo.Table2 
  ADD CONSTRAINT Num_and_Dat_are_Null_Together 
  CHECK ( Num IS NULL AND Dat IS NULL 
       OR Num IS NOT NULL AND Dat IS NOT NULL
        ) ;

Related Solutions

Sql-server – Why aren’t primary key / foreign key matches used for joins

In many cases, there are more than one way to join two tables; See the other answers for lots of examples. Of course, one could say that it would be an error to use the 'automatic join' in those cases. Then only a handfull of simple cases where it can be used would be left.

However, there is a severe drawback! Queries that are correct today, might become an error tomorrow just by adding a second FK to the same table!

Let me say that again: by adding columns, queries that do not use those columns could turn from 'correct' into 'error'!

That is such a maintenance nightmare, that any sane style guide would prohibit to use this feature. Most already prohibit select * for the same reason!

All this would be acceptable, if performance would be enhanced. However, that's not the case.

Summarizing, this feature could be used in only a limited set of simple cases, does not increase performance, and most style guides would prohibit its usage anyway.

Therefor it is not supprising that most database vendors choose to spend their time on more important things.

PostgreSQL – insert/update violates foreign key constraints

There are a few problems with your tables. I'll try to address the foreign keys first, since you question asked about them :)

But before that, we should realize that the two sets of tables (the first three you created and the second set, which you created after dropping the first set) are the same. Of course, the definition of Table3 in your second attempt has syntax and logical errors, but the basic idea is:

CREATE TABLE table3 (   
  "ID" bigint NOT NULL DEFAULT '0',   
  "DataID" bigint DEFAULT NULL,   
  "Address" numeric(20) DEFAULT NULL,   
  "Data" bigint DEFAULT NULL,
   PRIMARY KEY ("ID"),   
   FOREIGN KEY ("DataID") REFERENCES Table1("DataID") on delete cascade on update cascade,   
   FOREIGN KEY ("Address") REFERENCES Table2("Address") on delete cascade on update cascade
);

This definition tell PostgreSQL roughly the following: "Create a table with four columns, one will be the primary key (PK), the others can be NULL. If a new row is inserted, check DataID and Address: if they contain a non-NULL value (say 27856), then check Table1 for DataID˙and Table2 for Address. If there is no such value in those tables, then return an error." This last point which you've seen first:

ERROR: insert or update on table "Table3" violates foreign key constraint 
    "Table3_DataID_fkey" DETAIL: Key (DataID)=(27856) is not present in table "Table1".

So simple: if there is no row in Table1 where DataID = 27856, then you can't insert that row into Table3.

If you need that row, you should first insert a row into Table1 with DataID = 27856, and only then try to insert into Table3. If this seems to you not what you want, please describe in a few sentences what you want to achieve, and we can help with a good design.

And now about the other problems.

You define your PKs as

CREATE all_your_tables (
    first_column NOT NULL DEFAULT '0',   
    [...]
    PRIMARY KEY ("ID"),

A primary key means that all the items in it are different from each other, that is, the values are UNIQUE. If you give a static DEFAULT (like '0') to a UNIQUE column, you will experience bad surprises all the time. This is what you got in your third error message.

Furthermore, '0' means a text string, but not a number (bigint or numeric in your case). Use simply 0 instead (or don't use it at all, as I written above).

And a last point (I may be wrong here): in Table2, your Address field is set to numeric(20). At the same time, it is the PK of the table. The column name and the data type suggests that this address can change in the future. If this is true, than it is a very bad choice for a PK. Think about the following scenario: you have an address '1234567890454', which has a child in Table3 like

ID        DataID           Address             Data
123       3216547          1234567890454       654897564134569

Now that address happens to change to something other. How do you make your child row in Table3 follow its parent to the new address? (There are solutions for this, but can cause much confusion.) If this is your case, add an ID column to your table, which will not contain any information from the real world, it will simply serve as an identification value (that is, ID) for an address.

Best Answer

Related Solutions

Sql-server – Why aren’t primary key / foreign key matches used for joins

PostgreSQL – insert/update violates foreign key constraints

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