Q101. What does "locking" refer to?
A mechanism to prevent concurrent transactions from interfering with each other. Locks can be **shared** (read locks, multiple allowed) or **exclusive** (write locks, onl...
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Q102. Main transaction controls?
- `COMMIT` — Make changes permanent - `ROLLBACK` — Undo all changes in the transaction - `SAVEPOINT` — Set a partial rollback point ```sql START TRANSACTION; UPDATE accou...
Q103. A transaction that completes execution is said to be?
A database transaction that finishes all operations successfully and persists its changes permanently to durable storage is said to be **Committed**: ```sql START TRANSAC...
Q104. What does ROLLBACK do after COMMIT?
**`ROLLBACK` does nothing after a `COMMIT` has executed**. ### Why? Once a transaction issues `COMMIT`, the changes are permanently written to the database transaction lo...
Q105. Valid properties of a transaction?
The four foundational properties that guarantee database transactions are processed reliably are known as **ACID**: 1. **Atomicity**: "All or nothing." If any statement w...
Q106. What if autocommit is enabled?
Every individual statement is automatically committed. In MySQL, autocommit is ON by default. Turn it off for multi-statement transactions: ```sql SET autocommit = 0; -- ...
Q107. Default isolation level in MySQL?
**REPEATABLE READ** (InnoDB engine). Prevents dirty reads and non-repeatable reads, but phantom reads are possible (though InnoDB uses MVCC + gap locking to mostly preven...
Q108. Is autocommit enabled by default in MySQL?
**Yes**, in MySQL (using the standard InnoDB storage engine), **autocommit is enabled by default** (`autocommit = 1`): ### What Autocommit Means: - Every individual SQL s...
Q109. What is a virtual table in MySQL?
In MySQL and relational databases, a **virtual table** is called a **View**: ```sql CREATE VIEW active_customer_summary AS SELECT id, first_name, last_name, email FROM Cu...
Q110. Are views updatable?
**Some views are updatable, but under strict structural criteria**: ### Conditions for a View to Be Updatable: 1. Must reference **only one base table** in the `FROM` cla...