400 MySQL Interview Questions with Answers 2026

Advanced SQL Queries & Data Manipulation: CTEs, Window Functions, and complex Joins.

Performance Tuning & Query Optimization: Indexing strategies, execution plans, and partitioning.

Architecture & Reliability: ACID properties, storage engines, replication, and locking.

Security & Administration: SQL injection prevention, user privileges, and backups.

B) A transaction re-reads data and finds new rows added by a committed transaction in the interim.

C) A transaction reads a row that is subsequently deleted by another transaction.

D) The database fails to return any results due to a deadlock.

E) A transaction reads cached data that is no longer present on the disk.

F) Two transactions update the same row simultaneously, causing a lost update.

Correct Answer: B

Overall Explanation: Phantom reads occur when a transaction executes a range query twice, and the second result set includes "phantom" rows that were inserted and committed by another transaction during the interval.

Option Explanations:

B: Correct; this is the classic definition of a Phantom Read.

C: Incorrect; while related to consistency, this is typically categorized under non-repeatable reads.

D: Incorrect; deadlocks are a locking issue, not an isolation phenomenon.

E: Incorrect; this refers to cache-consistency issues, not SQL isolation levels.

F: Incorrect; this describes a "Lost Update" scenario.

Question 2: When optimizing a slow query in MySQL, what does the 'Using filesort' value in the 'Extra' column of an EXPLAIN output indicate?

B) MySQL is performing an external sort on disk because the data is too large for the buffer.

C) MySQL must perform an extra pass to find how to retrieve the rows in sorted order because it couldn't use an index.

D) The query is utilizing a temporary table created on the disk for sorting.

E) The index used for the query is corrupted and needs to be rebuilt.

F) The query is using the default primary key for sorting.

Correct Answer: C

Overall Explanation: 'Using filesort' means MySQL cannot use an index to satisfy an ORDER BY clause and must perform a manual sort of the result set.

Option Explanations:

B: Incorrect; filesort can happen in memory (sort_buffer_size) or on disk, but the term itself refers to the algorithm, not the location.

C: Correct; it signifies that an index-based sort was not possible.

D: Incorrect; that would be 'Using temporary'.

E: Incorrect; it is a performance indicator, not a corruption error.

F: Incorrect; if it used the primary key for sorting, 'Using filesort' would not appear.

Question 3: Which InnoDB locking mechanism is primarily used to prevent "Phantom Reads" in the REPEATABLE READ isolation level?

B) Table-level Metadata Locks

C) Gap Locks (Next-Key Locking)

D) Intention Exclusive (IX) Locks

E) Auto-increment Locks

F) Shared (S) Locks

Correct Answer: C

Overall Explanation: MySQL uses Next-Key Locking, which combines record locks and gap locks, to lock the "gaps" between index records, preventing other sessions from inserting new rows into those spaces.

Option Explanations:

B: Incorrect; metadata locks prevent schema changes, not DML inconsistencies.

C: Correct; Gap/Next-Key locks are the specific mechanism for preventing phantoms.

D: Incorrect; IX locks indicate a transaction intends to lock individual rows but don't prevent inserts.

E: Incorrect; these are used specifically for managing AUTO_INCREMENT values.

F: Incorrect; shared locks allow multiple reads but do not specifically address the phantom read gap issue.

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