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

Understanding Primary Keys in MySQL

A primary key is a column or a combination of columns that uniquely identifies each row in a table. It ensures that no two rows have the same value in the primary key column(s) and that the value is never NULL.

1. Characteristics of a Primary Key
  1. 1

    Uniquely identifies each row in the table.

  2. 2

    Cannot contain NULL values.

  3. 3

    Automatically creates a unique index.

  4. 4

    Ensures entity integrity in relational databases.

2. Can a Table Have More Than One Primary Key?
  1. 1

    A table can only have one primary key constraint.

  2. 2

    However, that primary key can consist of multiple columns — this is called a composite primary key.

  3. 3

    You cannot define two separate primary keys in one table.

Example: Composite Primary Key

In this example, neither student_id nor course_id alone uniquely identifies a row, but the combination does — forming a composite primary key.