# 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

```sql
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

```sql
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

```sql
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.

```sql
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).

```sql
START TRANSACTION;
```

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

```sql
COMMIT;
```

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

```sql
ROLLBACK;
```

Querying the table shows the committed change persists.

```sql
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.

```sql
SAVEPOINT  sv_update;
```

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

```sql
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).

```sql
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.

```sql
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.

```sql
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           |