Set the isolation level to 1 for the Accountant's connection by executing the following statement:

SET TEMPORARY OPTION isolation_level = 1;

Set the isolation level to 1 in the Sales Manager's window by executing the following statement:

SET TEMPORARY OPTION isolation_level = 1;

As the Accountant, execute the following statement to list the prices of the visors:

SELECT ID, Name, UnitPrice 
FROM GROUPO.Products;

As the Sales Manager, execute the following statements to introduce a new sale price for the plastic visor:

SELECT ID, Name, UnitPrice FROM GROUPO.Products
WHERE Name = 'Visor';
UPDATE GROUPO.Products
SET UnitPrice = 5.95 WHERE ID = 501;
COMMIT;
SELECT ID, Name, UnitPrice FROM GROUPO.Products
WHERE Name = 'Visor';

Compare the price of the visor in the Sales Manager window with the price for the same visor in the Accountant window. As the Accountant, execute the SELECT statement again and see the Sales Manager's new sale price:

SELECT ID, Name, UnitPrice
FROM GROUPO.Products;

This inconsistency is called a non-repeatable read because after executing the same SELECT a second time in the same transaction, the Accountant did not get the same results.

Of course, if the Accountant had finished the transaction, for example by issuing a COMMIT or ROLLBACK statement before using SELECT again, it would be a different matter. The database is available for simultaneous use by multiple users and it is completely permissible for someone to change values either before or after the Accountant's transaction. The change in results is only inconsistent because it happens in the middle of the transaction. Such an event makes the schedule unserializable.

The Accountant notices this behavior and decides that from now on he doesn't want the prices changing while he looks at them. Non-repeatable reads are eliminated at isolation level 2. As the Accountant, execute the following statements:

SET TEMPORARY OPTION isolation_level = 2;
SELECT ID, Name, UnitPrice
FROM GROUPO.Products;

The Sales Manager decides that it would be better to delay the sale on the plastic visor until next week so that she won't have to give the lower price on a big order that she's expecting to arrive tomorrow. As the Sales Manager, try to execute the following statements. The statement starts to execute, and then the window appears to freeze.

UPDATE GROUPO.Products
SET UnitPrice = 7.00
WHERE ID = 501;

The database server must guarantee repeatable reads at isolation level 2. Because the Accountant is using isolation level 2, the database server places a read lock on each row of the Products table that the Accountant reads. When the Sales Manager tries to change the price back, her transaction must acquire a write lock on the plastic visor row of the Products table. Since write locks are exclusive, her transaction must wait until the Accountant's transaction releases its read lock.

The Accountant is finished looking at the prices. He doesn't want to risk accidentally changing the database, so he completes his transaction with a ROLLBACK statement.

ROLLBACK;

When the database server executes this statement, the Sales Manager's transaction completes.

The Sales Manager can finish her transaction now. She wants to commit her change to restore the original price:

COMMIT;