The FOREIGN KEY constraint differs from the PRIMARY KEY constraint in that, you can create only one PRIMARY KEY per each table, with the ability to create multiple FOREIGN KEY constraints in each table by referencing multiple parent table.
Can one table have 2 foreign keys?
A table may have multiple foreign keys, and each foreign key can have a different parent table. Each foreign key is enforced independently by the database system.
How many foreign keys can be there in a table in SQL Server?
A table can reference a maximum of 253 other tables and columns as foreign keys (outgoing references).
How many foreign keys can the table have?
A table with a foreign key reference to itself is still limited to 253 foreign key references. Greater than 253 foreign key references are not currently available for columnstore indexes, memory-optimized tables, Stretch Database, or partitioned foreign key tables.
Can a table have multiple primary keys can a table have multiple foreign keys?
That’s impossible. A FOREIGN KEY constraint can only point to one table and each table can only have one PRIMARY KEY constraint. Or you can have multiple FOREIGN KEY constraints on the same column(s) referencing one PRIMARY KEY of a (different) table each. (Rarely useful.)
Can a table have multiple keys?
It is possible for a table to have multiple candidate keys , which effectively behave similar to a primary key in that a candidate key is unique, NOT NULL , and is a singular representation of that table record.
Can a table have both primary key and foreign key?
Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).
Can you have three foreign keys?
A table can have multiple foreign keys and no composite keys. A composite key simply means that there are two or more columns making up the key value.
How can I have two foreign keys in SQL?
You can use the FOREIGN KEY REFERENCES constraint to implement a foreign key relationship in SQL Server. Specify the table name. Then specify in parenthesis the column name for the foreign key to reference it.
How many secondary keys can be there in a table?
Non-unique secondary sorted keys defined using Non-Unique Sorted that are linked to table rows using a secondary table index (in which the system sorts the key fields in ascending order). An internal table can have up to 15 secondary table keys with different names.
Can a table have only foreign key?
There is no problem having a table that consists of foreign keys only. In your particular example it would be OK to remove the OrderID. Relational theory states that each table should have a candidate key so, ideally, the business rule you mention would be in place to make the foreign key only table a “good” table.
Can a table have 3 primary keys?
A table can only ever have a one primary key. It is not possible to create a table with two different primary keys. You can create a table with two different unique indexes (which are much like a primary key) but only one primary key can exist.
Can a foreign key have duplicate values?
Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them to contain NULL values. Though not automatically created for foreign keys, it is a good idea to define them. You can define several foreign key within a table.
Why can a table have only one primary key?
The major reason is because that is the definition of the primary key. A table can have multiple unique keys that identify each row, but only one primary key. In databases such as MySQL, the primary key is also a clustered index. That provides a more direct reason.