When a table contains a column (or concatenation of columns) that is the same as the primary key of a table, the column is called a foreign key. The matching of foreign key values to primary key values represents data relationships in a relational database.


Can we delete foreign key? Yes, you can. But you need to delete the foreign key constraint first in the database. Without the relationship constraints, you can pretty much drop the entire table containing the primary key as there are no more dependencies.

can a field be a primary and foreign key?

Primary keys always need to be unique, foreign keys need to allow non-unique values if the table is a one-to-many relationship. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.

Can foreign key have duplicate values? Unlike primary keys, foreign keys can contain duplicate values. Also, it is OK for them contain NULL values. Indexes aren't automatically created for foreign keys; however, as a DBA, you can define them. A table is allowed to contain more than one foreign key.

what is the difference between primary and foreign key?

Relation of Primary Key vs Foreign Key A primary key uniquely identifies a record in the relational database table, whereas a foreign key refers to the field in a table which is the primary key of another table.

Can we create foreign key without primary key? Yes. We can create a Foreign key without having Primary key in the parent table. Most of us are in a misconception that we can not create a Foreign key without primary key in the parent table. That unique id can be primary key or unique key.

can a primary key be a foreign key in the same table?

Creating a foreign key is almost as easy as creating a primary key, except that SQL Server imposes several more rules on foreign keys. For example, the foreign key must reference a primary key or unique constraint, although that reference can be on the same table or on a different table.

Can a table have no primary key? No. Every table should have some column (or set of columns) that uniquely identifies one and only one row. It makes it much easier to maintain the data. It's true, without a primary key (or some unique key), you don't have an insertion anomaly if you go to insert the same data multiple times.

What is primary key and foreign key with example?

A FOREIGN KEY is a key used to link two tables together. A FOREIGN KEY is a field (or collection of fields) in one table that refers to the PRIMARY KEY in another table. The "PersonID" column in the "Persons" table is the PRIMARY KEY in the "Persons" table.

How many primary keys can a table have? one primary key

Can a 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. Therefore, cascading relationships between tables can be established using foreign keys.

Can foreign key 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.

Can two tables share a primary key?

Yes, they can. But whether that's a good idea depends on how the data is used. Generally if two tables have the same primary key, it's because the data within the tables refer to the same object.

What is foreign key in DBMS?

A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. The concept of referential integrity is derived from foreign key theory. Foreign keys and their implementation are more complex than primary keys.

What will happen if we try to insert the same set of data again into a table which has primary key?

If you attempt to insert a row with the same primary key as a previous row, you will get a SQL error (try it in the commented out code below). If you insert a row without specifying the primary key, then SQL will automatically pick one for you that's different from other values.

Can foreign key refer to two tables?

A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. A foreign key acts as a cross-reference between tables in that it references the primary key or unique key columns of another table, and thus establishes a link between them.

How many foreign keys can a table have?

For one column, you can have upto 16 foreign keys. For one table, the current recommended number is 253 although you will be limited (forced) by the performance issues before you reach that number.

What is a foreign key example?

A foreign key is a column (or columns) that references a column (most often the primary key) of another table. For example, say we have two tables, a CUSTOMER table that includes all customer data, and an ORDERS table that includes all customer orders.

What is composite key in SQL?

A composite key is a combination of two or more columns in a table that can be used to uniquely identify each row in the table when the columns are combined uniqueness is guaranteed, but when it taken individually it does not guarantee uniqueness.