A collection of SQL queries that are treated as one unit of work
When you need to do one or more queries to achieve what you logically want in your application
Logically one thing that cannot be split
Can change data / be read-only
Example: Account deposit (3 queries)
- SELECT : Check if there’s enough money from the 1st account
- UPDATE: Deduct $100 from thyat account
- UPDATE Deposit the money in the other account (+ $100)
- Diagram

Transaction Lifespan

Databases implement all these things differently << Think about how dbs might have implemented things (original thoughts!)
Transaction BEGIN
- A transaction always starts with BEGIN
Transaction COMMIT
- Actually commits all the changes I made → makes it persist in the database
- Think about the work the db is actually doing to persist these changes
- As a student, how would you build this??
- Always think what is happening behind the scene!
- What if during commits, it crashes??
Transaction ROLLBACK
- Just flush anything in the memory ⇒Undo/Redo space
- Transaction unexpected ending = ROLLBACK (crash)

Nature of Transactions

Usually transactions are used to change and modify data
But you can also have a read only transaction → to maintain consistency
- You want to generate a report and you want to get consistent snapshot based at the time of transaction
- If anything changes during the read-only transaction, it’s isolated so it’s ok