y Reads A dirty read occurs when a transaction reads data that has not yet been committed. For example, suppose transaction 1 updates a ROW. Transaction 2 reads the updated row before transaction 1 commits the update. If transaction 1 rolls back the change, transaction 2 will have read data that is considered NEVER to have existed.

Nonrepeatable Reads A nonrepeatable read occurs when a transaction reads the same row twice but gets different data each time. For example, suppose transaction 1 reads a row. Transaction 2 updates or deletes that row and commits the update or delete. If transaction 1 rereads the row, it retrieves different row values or discovers that the row has been deleted.

Phantoms A PHANTOM is a row that matches the search criteria but is not initially SEEN. For example, suppose transaction 1 reads a set of rows that satisfy some search criteria. Transaction 2 generates a NEW row (through either an update or an insert) that matches the search criteria for transaction 1. If transaction 1 reexecutes the statement that reads the rows, it gets a different set of rows.