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How do you know when to use a foreign key?

A Foreign Key is a database key that is used to link two tables together. The FOREIGN KEY constraint identifies the relationships between the database tables by referencing a column, or set of columns, in the Child table that contains the foreign key, to the PRIMARY KEY column or set of columns, in the Parent table.

The only and most important difference between the two keywords ‘FOREIGN KEY” and “REFERENCES” keywords is though both of them make the data to be child data of the parent table, the “FOREIGN KEY” is used to create a table level constraint whereas REFERENCES keyword can be used to create column level constraint only.

Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key. (The data foreign key column must exist in the primary key column in the first table.)

A primary key is used to ensure data in the specific column is unique. A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table.

A primary key constrain is a column or group of columns that uniquely identifies every row in the table of the relational database management system. Foreign key is a column that creates a relationship between two tables. It helps you to uniquely identify a record in the table. It is a field in the table that is a primary key of another table.

Commonly used SQL Server Constraints: FOREIGN KEY, CHECK and DEFAULT. October 31, 2017. In the previous article Commonly used SQL Server Constraints: NOT NULL, UNIQUE and PRIMARY KEY, we described, in detail, the first three types of the SQL Server constraints; NOT NULL, UNIQUE and PRIMARY KEY.

Here is the T-SQL for a typical foreign key relationship. We can replicate this table structure using a check constraint instead of the foreign key constraint by using the following T-SQL. Note that with the check constraint we do have to specify the list of values that make up the check constraint.

Foreign key is a column that creates a relationship between two tables. The purpose of the Foreign key is to maintain data integrity and allow navigation between two different instances of an entity. It acts as a cross-reference between two tables as it references the primary key of another table.