Is foreign key mandatory for a table?

Is foreign key mandatory for a table?

Since the purpose of the foreign key is to identify a particular row of referenced table, it is generally required that the foreign key is equal to the candidate key in some row of the primary table, or else have no value (the NULL value.). ... Foreign keys play an essential role in database design.

Does every table need a primary key and foreign key?

Every table can have (but does not have to have) a primary key. ... (The data foreign key column must exist in the primary key column in the first table.) Non-Identifying One-To-One — means that the column dragged to a second table will not be added to its primary key, and that the relationship is mandatory and unique.

Which table should have the foreign key?

child table The table that contains the foreign key is considered the child table, and the table that the foreign key references is the parent table. The foreign key restricts what data can be stored in the foreign key columns in the child table, based on the data in the referenced columns in the parent table.

How many tables are required for foreign key?

two tables Performance. Because the foreign key constraint requires per-row checks on two tables, statements involving foreign key or referenced columns can take longer to execute.

Can foreign key be null?

By default there are no constraints on the foreign key, foreign key can be null and duplicate. while creating a table / altering the table, if you add any constrain of uniqueness or not null then only it will not allow the null/ duplicate values.

Can a primary key be a 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 a table have 2 primary keys?

No. You cannot use more than 1 primary key in the table. for that you have composite key which is combination of multiple fields.

Can 2 tables have the same primary key?

Yes. You can have same column name as primary key in multiple tables. Column names should be unique within a table. A table can have only one primary key, as it defines the Entity integrity.

Can a table have two primary keys?

No. You cannot use more than 1 primary key in the table. for that you have composite key which is combination of multiple fields.

Can foreign key be NULL?

By default there are no constraints on the foreign key, foreign key can be null and duplicate. while creating a table / altering the table, if you add any constrain of uniqueness or not null then only it will not allow the null/ duplicate values.

How to create a table with a foreign key?

• To create a table containing the following field names and data types: The table contains a PRIMARY KEY CONSTRAINT on 'cust_code' and a FOREIGN KEY on 'agent_code' without specifying the PRIMARY KEY column - The 'agent_code' in 'agents' table are unique.

What is the purpose of a foreign key in SQL?

• The main purpose of FOREIGN KEY is, only those values will appear which are present in the primary key table. For each row in the referencing table ( the table contains the FOREIGN KEY), the foreign key must match an existing primary key in the referenced table (the table contains the PRIMARY KEY).

Can a foreign key be a primary key?

• Foreign keys may be part of a concatenated primary key, or they may not be part of their table's primary key at all. Consider, for example, a pair of simple Antique Opticals customers and orders relations: The customer number column in the orders table is a foreign key that matches the primary key of the customers table.

Which is the foreign key in the CUSTOMER table?

• In the following illustration, the CUSTOMER and ORDER tables share the CustID column. CustID is a primary key in the CUSTOMER table, CustID is a foreign key in the ORDER table.