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How do I create a unique foreign key in SQL?
To create a SQL foreign key constraint, the parent table should have primary key column or column with UNIQUE constraint. In this case, table Dept is parent table which has Primary key and will be referenced in child tables having foreign key.
Can a foreign key be null and unique?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table).
Is foreign key unique MySQL?
MySQL requires that foreign key columns be indexed; if you create a table with a foreign key constraint but no index on a given column, an index is created. Exception: NDB Cluster requires an explicit unique key (or primary key) on the foreign key column.
Can foreign key references unique key?
A foreign key can refer to either a unique or a primary key of the parent table.
What is a foreign key relationship?
A foreign key is an attribute that completes a relationship by identifying the parent entity. Foreign keys provide a method for maintaining integrity in the data (called referential integrity) and for navigating between different instances of an entity. Every relationship in the model must be supported by a foreign key.
What is an unique key?
A unique key is a set of one or more than one column/field of a table that uniquely identify a record in a database table. The UNIQUE KEY constraint makes sure all the values in a column are unique within the database. Just like a primary key, a unique key can also comprise of more than one column.
What is a foreign key in SQL Server?
In database terms, a foreign key is a column that is linked to another table‘s primary key field in a relationship between two tables. A foreign key is a type of constraint, and so if you want to create a foreign key in SQL Server, you’ll need to create a foreign key constraint.
How do I create a primary key?
To create a primary key In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design. In Table Designer, click the row selector for the database column you want to define as the primary key. Right-click the row selector for the column and select Set Primary Key.