Foreign key in MS SQL Server
Prerequisite – Primary key in MS SQL Server
SQL Server has different keys which serve a different purpose. In this article, the foreign key will be discussed in brief.
The foreign key has a similar purpose as the primary key yet, the foreign key is used for two tables. In a few cases, the foreign key is used for self-referencing a single table.
Foreign key :
A singular column or a set of columns of one table that uniquely identified by a singular column or a set of columns of another table is referred to as a foreign key.
constraint fk_constraint-name foreign key(col1, col2) references parent_table-name(col1, col2) (OR) foreign key(col1, col2) references parent_table-name(col1, col2)
A foreign key has two tables – parent table and child table. If a user wants to insert a column in a child table, the column has to be a part of the parent table otherwise, an error is displayed. In the syntax, the constraint term is not mandatory to use.
When a foreign key is mentioned in the query, the key automatically creates a referential constraint meaning a column can be inserted in the child table only if it is a part of the parent table.
Two tables named student (parent table) and marks (child table) are considered from the university database.
Table – Marks
If a user wants to insert a new column, the query is given as –
foreign key('rollno') references student('rollno') insert into marks ('name', 'rollno', 'marks') values('Naina, '111', '7.5')
An error is displayed as the roll number is already taken by a student. (Foreign key constraint). The foreign key does not allow the value to reoccur. To avoid such errors, the values must neither be repeated nor a different column must be considered.