What is a database index and how does it work?

MyISAM is a non-transactional storage engine, ideal for read-heavy applications, while InnoDB supports transactions, foreign keys, and row-level locking, making it suitable for high-concurrency environments. For instance, InnoDB is preferable for e-commerce sites where data integrity is critical.

To restore a MySQL database, use the command line with the `mysql` tool. For example, running `mysql -u username -p database_name < backup.sql` will restore the database from the specified backup file, recreating the original structure and data.

The AUTO_INCREMENT attribute allows MySQL to automatically generate a unique integer for a column, typically used for primary keys. For example, defining a column as `user_id INT AUTO_INCREMENT PRIMARY KEY` ensures each new user record gets a unique ID without manual input.

User-defined functions (UDFs) allow users to create custom functions to encapsulate reusable logic in SQL. UDFs can take parameters and return values. For example, a UDF to calculate tax could be defined as `CREATE FUNCTION CalculateTax(amount DECIMAL) RETURNS DECIMAL BEGIN RETURN amount * 0.1; END;`.

To check the performance of a MySQL query, use the `EXPLAIN` statement before your SELECT query. This provides insights into how MySQL executes the query, revealing details such as which indexes are used and the estimated number of rows processed. For example, `EXPLAIN SELECT * FROM users WHERE age > 30;` gives performance metrics.

The CASE statement allows conditional logic in SQL queries, returning values based on specified conditions. It works like an IF statement. For example, `SELECT name, CASE WHEN score >= 60 THEN 'Pass' ELSE 'Fail' END AS result FROM exams;` assigns 'Pass' or 'Fail' based on the score.

Stored procedures offer several advantages: they enhance performance by reducing the amount of data sent over the network, promote code reusability, encapsulate business logic, and improve security by limiting direct access to tables. For example, a stored procedure for processing orders can manage all related SQL operations.

TRUNCATE is used to remove all rows from a table quickly without logging individual row deletions. It resets any AUTO_INCREMENT values to zero. For example, executing `TRUNCATE TABLE orders;` removes all orders in a fraction of the time compared to DELETE, but cannot be rolled back.

Database migration involves transferring data between different database systems or versions. This can be achieved using tools like `mysqldump` for exporting and importing data or third-party migration tools. For instance, exporting a database with `mysqldump` and importing it to a new server using `mysql` command facilitates migration.

A database index is a data structure that improves the speed of data retrieval operations. It works by creating a sorted representation of the indexed column(s). For instance, indexing the 'email' column allows for quick lookups of user accounts based on their email addresses.