[Solved] You execute this PostgreSQL code:BEGIN;UPDATE wallets SET amount = amount - 50 WHERE id = 2;SELECT amount FROM wallets WHERE id = 2;ROLLBACK;SELECT amount FROM wallets WHERE id = 2;What value does... | PostgreSQL

PostgreSQL - Transactions and Concurrency

You execute this PostgreSQL code:

BEGIN;
UPDATE wallets SET amount = amount - 50 WHERE id = 2;
SELECT amount FROM wallets WHERE id = 2;
ROLLBACK;
SELECT amount FROM wallets WHERE id = 2;

What value does the second SELECT return?

AThe decreased amount after update

BThe original amount before the update

CNULL because the transaction was rolled back

DAn error due to rollback

Step-by-Step Solution

Solution:

Step 1: Understand transaction effects
The UPDATE reduces amount but is not committed.
Step 2: Effect of ROLLBACK
ROLLBACK undoes the update, restoring original data.
Step 3: Final SELECT after rollback
Returns original amount as update was discarded.
Final Answer:
The original amount before the update -> Option B
Quick Check:
Rollback restores original data [OK]

Quick Trick: Rollback discards uncommitted changes [OK]

Common Mistakes:

Expecting updated value after rollback
Assuming rollback causes NULL result
Thinking rollback triggers error on SELECT

Master "Transactions and Concurrency" in PostgreSQL

9 interactive learning modes - each teaches the same concept differently

Learn Why Deep Visual Try Challenge Project Recall Time

You execute this PostgreSQL code:

Step 1: Understand transaction effects

Step 2: Effect of ROLLBACK

Step 3: Final SELECT after rollback

Final Answer:

Quick Check:

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