Transaction isolation levels in SQL - Step-by-Step Execution

Concept Flow - Transaction isolation levels

Start Transaction

↓

Set Isolation Level

↓

Execute Queries

↓

Check for Conflicts

↓

No Conflicts

↓

Commit

↓

End Transaction

This flow shows how a transaction starts, sets its isolation level, executes queries, checks for conflicts or read anomalies, and then commits or rolls back accordingly.

Execution Sample

SQL

BEGIN TRANSACTION;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SELECT * FROM accounts WHERE id = 1;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
COMMIT;

A transaction starts with the highest isolation level, reads data, updates it, and commits to ensure no conflicts or anomalies.

Execution Table

Step	Action	Isolation Level	Data Read	Data Written	Conflict Check	Result
1	BEGIN TRANSACTION	Default (Read Committed)	None	None	None	Transaction started
2	SET ISOLATION LEVEL SERIALIZABLE	Serializable	None	None	None	Isolation level set
3	SELECT accounts WHERE id=1	Serializable	Row with id=1, balance=500	None	None	Data read with full isolation
4	UPDATE accounts SET balance=balance-100 WHERE id=1	Serializable	Same row	Balance updated to 400	Check for write conflicts	No conflicts, update allowed
5	COMMIT	Serializable	None	None	Final conflict check	Transaction committed successfully

💡 Transaction ends after commit with no conflicts under SERIALIZABLE isolation level

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
Isolation Level	Read Committed	Serializable	Serializable	Serializable
Data Read	None	Row(id=1, balance=500)	Row(id=1, balance=500)	None
Data Written	None	None	Row(id=1, balance=400)	None
Transaction State	Not started	Active	Active	Committed

Key Moments - 3 Insights

Why does the data read at step 3 show balance=500 even though it will be updated later?

Because the SELECT reads the current committed data before the UPDATE changes it, as shown in execution_table row 3 and 4.

What happens if a write conflict is detected at step 4?

The transaction would either wait or rollback to avoid dirty writes, but in this example no conflict occurs (row 4).

Why is the isolation level set before executing queries?

Setting isolation level first ensures all queries in the transaction follow the same rules to prevent anomalies, as shown in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the isolation level after step 2?

A Repeatable Read

B Read Committed

C Serializable

D Read Uncommitted

💡 Hint

Check the Isolation Level column at step 2 in the execution_table.

At which step is the data balance updated to 400?

A Step 4

B Step 3

C Step 5

D Step 2

💡 Hint

Look at the Data Written column in the execution_table.

If the isolation level was Read Uncommitted, what anomaly might occur during the SELECT at step 3?

A No anomaly

B Dirty Read

C Phantom Read

D Write Conflict

💡 Hint

Consider what Read Uncommitted allows compared to Serializable in the concept_flow.

Concept Snapshot

Transaction Isolation Levels:
- Control how transactions see and affect data
- Levels: Read Uncommitted, Read Committed, Repeatable Read, Serializable
- Higher levels prevent more anomalies but reduce concurrency
- Set isolation level at transaction start
- Commit or rollback ends transaction

Full Transcript

This visual execution shows a transaction starting, setting its isolation level to SERIALIZABLE, reading data, updating it, and committing. The isolation level controls how data is read and written to avoid conflicts and anomalies. The execution table tracks each step, showing the isolation level, data read and written, conflict checks, and results. Variables like isolation level and transaction state change as the transaction progresses. Key moments clarify why data reads show old values before updates and the importance of setting isolation level early. The quiz tests understanding of isolation levels and their effects on data visibility and conflicts.