The InnoDB Storage Engine is responsible for handling transactions and ensuring ACID (Atomicity, Consistency, Isolation, Durability) compliance in MySQL. It provides features like row-level locking, transaction isolation levels, and crash recovery, making it the preferred storage engine for transactional workloads.

A LEFT JOIN returns all rows from the left table and the matching rows from the right table. If there is no match, the result will still include rows from the left table with NULL values for columns from the right table.

VARCHAR is the best datatype for storing a social security number because it often contains non- numeric characters such as hyphens, and the number itself is not used in calculations.

The innodb_log_file_size parameter controls the size of the redo log files, which are essential for recovering transactions in the event of a crash. The larger the log file, the more data can be stored before it is flushed to disk.

The Password_expired column in the mysql.user table specifies whether a user must change their password upon the next login. This is typically used to enforce password rotation policies.