Final answer:
The default transaction isolation level is READ COMMITTED, but if READ UNCOMMITTED is specified, READ ONLY is assumed. Isolation levels like READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE define how transactions interact in database systems.
Step-by-step explanation:
The default transaction behavior is READ COMMITTED unless the isolation level of READ UNCOMMITTED is specified, in which case READ ONLY is assumed. In database systems, isolation levels are used to define the degree to which the transactions are isolated from each other. This impacts how and when the changes made by one transaction are visible to others. The standard isolation levels include READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE, with each level providing a different balance between performance and data accuracy.
When the READ UNCOMMITTED isolation level is specified, it allows a transaction to read data that has not yet been committed by other transactions, which may lead to dirty reads. Conversely, READ COMMITTED is a more restrictive isolation level that only allows a transaction to read data that has been committed, thereby preventing dirty reads.