Editing Isolation (database systems) (section)

===Non-repeatable reads===
A ''non-repeatable read'' occurs when a transaction retrieves a row twice and that row is updated by another transaction that is committed in between.

In this example, transaction 1 retrieves the row with id 1, then transaction 2 updates the row with id 1 and is committed, and finally transaction 1 retrieves the row with id 1 again. At the READ UNCOMMITTED and READ COMMITTED isolation levels, the second SELECT in transaction 1 retrieves the updated row: this is a non-repeatable read. At the REPEATABLE READ and SERIALIZABLE isolation levels, the second SELECT in transaction 1 retrieves the initial row.

{|
|-
! Transaction 1
! Transaction 2
|-
|<syntaxhighlight lang="sql">
BEGIN;
SELECT age FROM users WHERE id = 1;
-- retrieves 20
</syntaxhighlight>
|
|-
|
|<syntaxhighlight lang="sql">
BEGIN;
UPDATE users SET age = 21 WHERE id = 1;
COMMIT;
</syntaxhighlight>
|-
|<syntaxhighlight lang="sql">
SELECT age FROM users WHERE id = 1;
-- READ UNCOMMITTED retrieves 21 (non-repeatable read)
-- READ COMMITTED retrieves 21 (non-repeatable read)
-- REPEATABLE READ retrieves 20 (non-repeatable read has been avoided)
-- SERIALIZABLE retrieves 20 (non-repeatable read has been avoided)
COMMIT;
</syntaxhighlight>
|}