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SQL Transaction Control

Transaction Control Language (TCL) manages transactions within a database, ensuring data consistency and reliability. A transaction is a logical unit of work comprising one or more SQL statements, executed co-relationally against data.

TCL Commands

• COMMIT
• ROLLBACK
• SAVEPOINT

COMMIT

The COMMIT command final izes a transaction. All data modifications made within the transaction become permanent and visible to other database sessions. Executing COMMIT guarantees data persistence up to that point.

Syntax

COMMIT;

ROLLBACK

ROLLBACK discards all changes made since the transaction started or since the last SAVEPOINT. It restores the database to a previous state, effectively undoing operations within the transaction. This is crucial for error handling and partial transaction reversal.

Syntax

ROLLBACK TO savepoint-name;

Note: Without the TO savepoint-name clause, ROLLBACK undoes the entire current transaction.

SAVEPOINT

A SAVEPOINT establishes a marker within a transaction. You can ROLLBACK to a specific savepoint, partially undoing changes while retaining prior modifications within the same transaction. This allows fine-grained control over transaction scope.

Syntax

SAVE POINT savepoint-name;

Examples

Using the purchase table to demonstrate TCL command behavior.

item	price	customer_name
Pen	10	Troy
Bag	1000	Troy
Vegetables	500	Troy
Shoes	5000	Troy
Water Bottle	800	XYZ
Mouse	120	ABC
Sun Glasses	1350	ABC

Update the price of "Pen" from 10 to 20.

UPDATE purchase SET price = 20 WHERE item = "Pen";

Output indicates success. The change is applied (though not necessarily permanent yet depending on database settings). ``` Query OK, 1 row affected (...)

View the updated table state.

```sql
SELECT * FROM purchase;

item	price	customer_name
Pen	20	Troy
Bag	1000	Troy
Vegetables	500	Troy
Shoes	5 000	Troy
Water Bottle	800	XYZ
Mouse	120	ABC
Sun Glasses	1350	ABC

Begin a transaction explicitly (some databases default to autocommit).

START TRANSACTION;

Commit the changes made so far within this explicit or implicit transaction. This makes the "Pen" price update (10 to 20) permanent.

COMMIT;

Attempting to rollback now has no effect on the committed transaction.

ROLLBACK;

Querying the table shows the committed change persists.

SELECT  * FROM purchase;

item	price	customer_name
Pen	20	Troy
Bag	1000	Troy
Vegetables	500	Troy
Shoes	5000	Troy
Water Bottle	800	XYZ
Mouse	120	ABC
Sun Glasses	1350	ABC

Create a savepoint named sv_update in the current transaction. Changes before this point are protected from rollbacks targeting this savepoint.

SAVEPOINT  sv_update;

Update the price of "Pen" again, from 20 to 30. This change occurs after the sv_update savepoint .

UPDATE purchase SET price = 30 WHERE item = "Pen";

Output indicates success.

Query OK, 1 row affected (...)

View the table state showing the new price (30).

SELECT * FROM purchase;

item	price	customer_name
Pen	30	Troy
Bag	1000	Troy
Vegetables	500	Troy
Shoes	5000	Troy
Water Bottle	800	XYZ
Mouse	120	ABC
Sun Glasses	1350	ABC

Rollback the transaction specifically to the sv_update savepoint. This undoes the price change from 20 to 30, but not the prior change from 10 to 20 because it was committed.

ROLLBACK to sv_update;

Querying the table shows the state after the partial rollback. The price of "Pen" is restored to 20, as this was the state at the sv_update savepoint.

SELECT * FROM purchase;

item	price	customer_name
Pen	20	Troy
Bag	1000	Troy
Vegetables	500	Troy
Shoes	5000	Troy
Water Bottle	800	XYZ
Mouse	120	ABC
Sun Glasses	1350	ABC