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 are candidate keys, and how do they differ from alternate keys?

Understanding Candidate Keys and Alternate Keys

In relational database design, candidate keys and alternate keys are essential concepts used to uniquely identify rows and enforce data integrity.

1. What Is a Candidate Key?
  1. 1

    A candidate key is any column or combination of columns that can uniquely identify a row in a table.

  2. 2

    A table can have multiple candidate keys.

  3. 3

    Among the candidate keys, one is chosen as the primary key.

  4. 4

    Candidate keys must be unique and cannot contain NULL values.

Example Table

In this table, emp_id, email, and phone could all uniquely identify a row. Therefore, all three are candidate keys.

2. What Is an Alternate Key?
  1. 1

    An alternate key is any candidate key that is not selected as the primary key.

  2. 2

    It still has the unique property, but it serves as an additional unique constraint.

  3. 3

    In the example above, if emp_id is the primary key, then email and phone become alternate keys.

Example: Defining Alternate Keys
3. Difference Between Candidate Key and Alternate Key
  1. 1

    Candidate Keys → All possible keys that can uniquely identify a record.

  2. 2

    Primary Key → One candidate key chosen as the main identifier.

  3. 3

    Alternate Keys → The remaining candidate keys that are not chosen as the primary key.

In summary, every alternate key is a candidate key, but not every candidate key becomes an alternate key.