Can a non primary key be foreign key?
Yes, a foreign key can reference a non-primary key which is unique. A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.
Is foreign key indexed?
No, there is no implicit index on foreign key fields, otherwise why would Microsoft say “Creating an index on a foreign key is often useful”. Your colleague may be confusing the foreign key field in the referring table with the primary key in the referred-to table – primary keys do create an implicit index.
What’s the difference between a primary key and a foreign key?
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. Its value cannot be deleted from the parent table.
What is a primary key and foreign key?
A primary key is a column or a set of columns in a table whose values uniquely identify a row in the table. A foreign key is a column or a set of columns in a table whose values correspond to the values of the primary key in another table.
Can a foreign key be another foreign key?
A foreign key can reference any field defined as unique. If that unique field is itself defined as a foreign key, it makes no difference. If it is a unique field, it can also be the target of another FK.
Is it mandatory for a foreign key to be a primary key in another table?
Yes, foreign key has to be primary key of parent table.
What is difference between index and foreign key?
an index on a table is a data structure that makes random access to the rows fast and efficient. It helps to optimize the internal organization of a table as well. A foreign key is simply a pointer to a corresponding column in another table that forms a referential constraint between the two tables.
Does every foreign key need an index?
It is highly recommended to create an index on each foreign key constraint on the child table, as it is very common when calling that key on your queries to join between the child table and the parent table columns, providing better joining performance.
Why foreign keys are not redundant?
Essentially, primary and foreign keys are used as a way to constrain or link related data in a database. This ensures that data remains consistent and that the database contains no redundant data. For example, if you delete a table (or even a row in a table) that other tables rely on, the redundant data is removed.
How to create a foreign key to a non primary key?
If you really want to create a foreign key to a non-primary key, it MUST be a column that has a unique constraint on it. A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table.
Can a foreign key automatically create an index?
A foreign key cannot automatically create an index, clustered, or non-clustered. You can have the single Primary key in a table. You can have multiple foreign keys in a table.
What’s the difference between unique key and foreign key?
Unique Constraint may have a NULL value. Each table can have more than one Unique Constraint. By default, Unique key is a unique non-clustered index. Unique Constraint can not be related with another table’s as a Foreign Key. Foreign key is a field in the table that is Primary key in another table.
Can a FOREIGN KEY constraint be linked to a primary key constraint?
A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table. So in your case if you make AnotherID unique, it will be allowed.