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Questions and Answers
What is the purpose of a primary key in a database table?
In a relational database, what is the role of a foreign key?
Which of the following statements about primary keys is true?
What happens if a foreign key value does not exist in the referenced primary key column?
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Which column would typically serve as a primary key in a table of 'Orders'?
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Study Notes
Database Keys
In the context of relational databases, a key is a column or set of columns that are used to establish and maintain relationships between tables. Two specific types of keys are primary keys and foreign keys.
Primary Key
A primary key is a column (or set of columns) in a table that uniquely identifies each row within the table. It ensures that no two rows have the same values in the primary key columns. Primary keys are essential for maintaining data integrity and are often used as a reference for foreign keys in other tables. A primary key cannot contain null values and must be unique across all rows in the table.
For example, in a table of Users, the UserId column could be a primary key. Each user would have a unique UserId that could be referenced in other tables, such as a Posts table, where the AuthorId column would hold the UserId of the user who wrote the post.
Foreign Key
A foreign key is a column (or group of columns) in a table that is used to establish a relationship between the current table and another table. It references the primary key of another table, creating a link between the two. Foreign keys ensure data consistency and enforce referential integrity by ensuring that the value in the foreign key column exists in the primary key column of the referenced table.
For example, in a Posts table, the AuthorId column could be a foreign key. It would reference the UserId column in the Users table, allowing the database to retrieve the name or other information associated with the user who wrote a specific post.
Primary Key vs. Foreign Key
The main difference between primary and foreign keys lies in their roles within a table. A primary key uniquely identifies each row within a table, while a foreign key creates a relationship between tables.
In summary, primary keys are used to ensure data uniqueness within a table, while foreign keys are used to establish relationships between tables. Both types of keys are crucial for maintaining data integrity in relational databases.
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Description
Learn about primary keys and foreign keys in the context of relational databases. Understand how primary keys uniquely identify rows within a table while foreign keys establish relationships between tables, ensuring data integrity and consistency.
