Stage 2 · Lesson 7 of 10

Transactions and ACID

A groups several statements so they all succeed or all fail together. Moving money between accounts is two updates: take it from one, add it to the other. Without BEGIN and COMMIT, a crash between them loses money.

ACID names the four guarantees. Atomic means all or nothing. Consistent means your constraints still hold.

Isolated means transactions running at the same time do not see each other's half-done work. Durable means committed data survives a crash. Keep transactions short.

Never wait on a network call or a user inside one, because locks are held until it ends. Frameworks usually open one per request or per unit of work.

Common trapreading a value, deciding in code and then writing: another request can change the row in between.

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Resources for this stage

Start with· follow one of these

CoursePaid
Grokking Relational Database Design and Modeling
Design Gurus · Intermediate · $98 one-time
The modelling and normalisation decisions that stay with a codebase for years
CourseFree
CS50's Introduction to Databases with SQL
Harvard University · Beginner
Free course from querying through schema design, indexes, transactions and scaling

Practise

Grokking SQL for Tech InterviewsPaid
Structured SQL practice on joins, grouping and subqueries once the basics feel easy
PostgreSQL Exercises
Free SQL drills on one real schema, from simple selects up to recursive queries

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