Can you have foreign keys across databases?

Can you have foreign keys across databases?

The short answer is that SQL Server (as of SQL 2008) does not support cross database foreign keys–as the error message states. While you cannot have declarative referential integrity (the FK), you can reach the same goal using triggers.

Can a row have multiple foreign keys?

No. That is, you cannot create a foreign key constraint this way. You can however, use a foreign key without a foreign key constraint. All a foreign key is, is the value of another table’s (or another record in the same table) primary key, which can be used in joins.

How do I add a foreign key to another database?

Using SQL Server Management Studio

1. In Object Explorer, right-click the table that will be on the foreign-key side of the relationship and click Design.
2. From the Table Designer menu, click Relationships.
3. In the Foreign-key Relationships dialog box, click Add.
4. Click the relationship in the Selected Relationship list.

What is the purpose of referential integrity?

Referential integrity refers to the relationship between tables. Because each table in a database must have a primary key, this primary key can appear in other tables because of its relationship to data within those tables. When a primary key from one table appears in another table, it is called a foreign key .

Can foreign keys be NULL?

A foreign key containing null values cannot match the values of a parent key, since a parent key by definition can have no null values. However, a null foreign key value is always valid, regardless of the value of any of its non-null parts. A foreign key value is null if any part is null.

Can a foreign key reference multiple columns?

MySQL allows us to add a FOREIGN KEY constraint on multiple columns in a table. The condition is that each Foreign Key in the child table must refer to the different parent table.

How to use foreign key in multiple tables?

If they should be modeled as different tables and have different columns, introduce two foreign key columns in fields, ideally with a check constraint: Then you would reference project_or_invoice in your foreign key. Thanks for contributing an answer to Database Administrators Stack Exchange!

Can a database have a foreign key reference?

Cross-database foreign key references are not supported. Foreign key Database2.table2. Could not create constraint. See previous errors. How do I do that since the tables are in different databases. You would need to manage the referential constraint across databases using a Trigger.

Can a cross database foreign key be used in SQL Server?

It can be disabled and reenabled when necessary same as foreign keys and rechecked after a database2 restore. The short answer is that SQL Server (as of SQL 2008) does not support cross database foreign keys–as the error message states. While you cannot have declarative referential integrity (the FK), you can reach the same goal using triggers.

When to use FK constraint between two databases?

If you need rock solid integrity, have both tables in one database, and use an FK constraint. If your parent table is in another database, nothing prevents anyone from restoring that parent database from an old backup, and then you have orphans. This is why FK between databases is not supported.