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What is the difference between primary key and foreign key in SQL?
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.
Why is it essential to use a primary key and foreign keys in a relational database?
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.
Can a table have no foreign key?
You don’t have to configure a foreign key constraint on a column just because it refers to another column. You could instead configure two tables such that one refers to the other, but without any defined foreign key. You could add a UserId column to Customer and still use it for querying purposes.
Can a FOREIGN KEY constraint improve query performance?
In case of reading the query, the optimizer can use foreign key constraints to create more efficient query plans as foreign key constraints are pre declared rules.
Why does a primary key speed up an index?
It has an associated index, for fast query performance. Query performance benefits from the NOT NULL optimization, because it cannot include any NULL values. With the InnoDB storage engine, the table data is physically organized to do ultra-fast lookups and sorts based on the primary key column or columns.
What does a primary key for a table mean?
The primary key for a table represents the column or set of columns that you use in your most vital queries. It has an associated index, for fast query performance. Query performance benefits from the NOT NULL optimization, because it cannot include any NULL values.
What is the purpose of a foreign key in SQL?
A foreign key is a DBMS concept for ensuring database integrity. Any performance implications/improvements will be specific to the database technology being used and are secondary to the purpose of a foreign key. It is good practice in SQL Server to ensure that all foreign keys have at least a non clustered index on them.