In MySQL, what is the primary difference between the MyISAM and InnoDB storage engines in terms of transactions and locking?

Introduction / Context:
MySQL offers several storage engines, with MyISAM and InnoDB being two of the most historically important. Choosing the right storage engine significantly impacts performance, data integrity, and concurrency. Interviewers often ask about the difference between MyISAM and InnoDB because it reveals whether you understand foundational trade offs such as transaction support, locking behaviour, and referential integrity in relational databases.

Given Data / Assumptions:

• We are using MySQL or MariaDB with support for multiple storage engines.
• Tables can be created using either MyISAM or InnoDB as the storage engine.
• ACID transactions, foreign key constraints, and locking granularity are important design factors.
• The question focuses on core differences, not on every minor feature.

Concept / Approach:
InnoDB is a transactional storage engine that supports ACID properties, row-level locking, and foreign key constraints. This makes it suitable for many OLTP workloads where data integrity and concurrent writes are important. MyISAM, in contrast, is a non transactional engine that does not support foreign keys and uses table-level locking. While MyISAM used to be popular for read heavy workloads due to its simplicity, it does not provide the same level of safety as InnoDB for critical data. Understanding these fundamental differences helps you decide which engine to use for specific tables or applications.

Step-by-Step Solution:
Step 1: Recognize that InnoDB supports transactions with COMMIT and ROLLBACK, allowing you to group multiple statements into an atomic unit of work. Step 2: Note that InnoDB enforces foreign key constraints, ensuring referential integrity between related tables at the database level. Step 3: Understand that InnoDB implements row-level locking, meaning only the rows being updated are locked, which improves concurrency under heavy write loads. Step 4: Contrast this with MyISAM, which does not support transactions or foreign keys and relies on table-level locking, locking the entire table for write operations. Step 5: Conclude that InnoDB is generally preferred for transactional applications, while MyISAM may still appear in older or read mostly scenarios but is not recommended for critical transactional data.

Verification / Alternative check:
MySQL documentation for InnoDB clearly lists ACID compliance, foreign key support, and row-level locking as core features. In documentation for MyISAM, you will see that transactions and foreign key constraints are not supported and that MyISAM uses table-level locking. Running test scripts that attempt to use START TRANSACTION or foreign keys on MyISAM tables will either fail or not behave as expected, while the same operations on InnoDB tables work correctly, confirming the difference in capabilities.

Why Other Options Are Wrong:
Option B incorrectly swaps capabilities, claiming MyISAM supports full ACID transactions and row-level locking, which it does not. Option C states that the engines are identical, contradicting documented feature differences. Option D suggests InnoDB is only for temporary tables, which is false; InnoDB is the default storage engine for most modern MySQL installations and is used for normal permanent tables.

Common Pitfalls:
A common pitfall is using MyISAM for tables that require strong data integrity, leading to data inconsistency after crashes because MyISAM lacks transactional guarantees. Another mistake is ignoring locking behaviour; table-level locking in MyISAM can cause serious contention under many concurrent writes. Best practice today is usually to default to InnoDB for application tables unless there is a very specific and well understood reason to use another engine.

Final Answer:
In MySQL, InnoDB supports transactions, foreign keys, and row-level locking, while MyISAM does not support transactions or foreign keys and relies on table-level locking.