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TOPIC OVERVIEW

mysql/Keys

Use keys to uniquely identify records and connect related tables while keeping your data consistent.

18 QuestionsIntermediateHigh Priority ~12 min read

Keys are an important part of relational database design. A primary key uniquely identifies a row, while foreign keys create relationships between tables and help maintain referential integrity.

MySQL also supports unique keys and composite keys for situations where one column alone is not enough to identify a record or enforce uniqueness. Choosing keys carefully affects both data correctness and query performance.

What you'll walk away knowing

What primary and foreign keys do

How unique keys enforce uniqueness

When composite keys are useful

How foreign key constraints protect relationships between tables

1What is a key in MySQL and why is it used?ExpertHigh 2What is the difference between a key and an index?ExpertHigh 3What is a primary key? Can a table have more than one primary key?ExpertHigh 4What is a foreign key and why is it important in relational databases?ExpertHigh 5How do you define a primary key when creating a table?ExpertHigh 6Can a primary key column contain NULL values? Why or why not?ExpertHigh 7What is the difference between a UNIQUE key and a PRIMARY key?ExpertHigh 8What is an AUTO_INCREMENT key, and how does it work with the primary key?ExpertHigh 9Can two tables have the same primary key name? Explain.ExpertHigh 10What is the purpose of the FOREIGN KEY constraint in maintaining referential integrity?ExpertHigh 11What is a composite key? How do you define one in MySQL?ExpertHigh 12What are candidate keys, and how do they differ from alternate keys?ExpertHigh 13What happens if you try to insert a duplicate value into a UNIQUE key column?ExpertHigh 14How do foreign key constraints behave on DELETE and UPDATE operations (CASCADE, SET NULL, etc.)?ExpertHigh 15What are the differences between ON DELETE CASCADE and ON DELETE SET NULL? What is a super key, and how does it relate to candidate keys? Can a foreign key reference a non-primary key column in another table?ExpertHigh 16How do you disable and re-enable foreign key checks in MySQL? Why would you do that?ExpertHigh 17How do you drop a primary key or foreign key from an existing table?ExpertHigh 18How can you find all foreign key relationships in a MySQL database?ExpertHigh 19How does InnoDB enforce foreign key constraints internally?ExpertHigh 20What is the impact of defining multiple UNIQUE keys on the same table?ExpertHigh 21Can you define a foreign key constraint referencing a table in a different database schema?ExpertHigh 22Explain how key definitions affect indexing and query optimization.ExpertHigh 23What are the differences between logical keys (like candidate or composite keys) and physical indexes?ExpertHigh 24Can a composite key contain a foreign key as one of its columns? Provide an example.ExpertHigh 25How does MySQL handle updates to parent keys that are referenced by foreign keys?ExpertHigh 26What are deferred constraint checks, and does MySQL support them? How do you design keys in a many-to-many relationship using a junction (bridge) table? What are the potential performance implications of using multiple foreign keys and composite keys in large-scale systems?ExpertHigh

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