Isolation levels

Last reviewed Jun 1, 2026 Content v20260601

Track mode

sql_sandbox

Means

SQL sandbox

Reading

~1 min

Level

advanced

This lesson

This lesson teaches Isolation levels: the SQL patterns, schema habits, and query reasoning you need before advancing in MySQL.

Teams query Isolation levels on every MySQL codebase—skipping it leaves gaps in debugging and data reviews.

You will apply Isolation levels in contexts like: Web apps on shared hosting, ecommerce, and many startups’ first production DB.

Copy MySQL SQL into the mysql client, local MySQL/MariaDB, or DB Fiddle (MySQL dialect)—use DESCRIBE and EXPLAIN where lessons show them. The in-browser lab ships later; mysql client is the practice path now.

When InnoDB, indexes, and EXPLAIN from intermediate lessons make sense in the mysql client.

READ COMMITTED and REPEATABLE READ (InnoDB default) trade consistency for concurrency—phantom reads possible depending on level.

Set session level

SELECT @@transaction_isolation;
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

Practice: Run read-only EXPLAIN on practice tables where possible.

InnoDB REPEATABLE READ

Default uses next-key locking to reduce phantoms—still understand anomalies for interview questions.

Important interview questions and answers

Q: READ UNCOMMITTED?
A: Rarely used—dirty reads possible.
Q: SERIALIZABLE?
A: Strictest—more locking, use when correctness demands.

Self-check

Default InnoDB isolation?
Why change isolation per session?

Tip: Know default REPEATABLE READ for InnoDB interviews.

Interview prep

InnoDB default?: REPEATABLE READ.
Dirty read?: Possible at READ UNCOMMITTED—not default.

Discussion

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Starter discussion topics

Default level?
Dirty reads?

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