| 1 | Read Uncommitted | The user A start a query, then user B made some changes, A will see the changes have not been committed yet (dirty read) |
| 2 | Read committed | The user A start a query, user B made some changes and commit. But A will not see the changes made by B. |
| 3 | Repeatable Read | The user A will not see the changes made by B (insert/delete) read the snapshot established by the first read, so 2 queries executed, with 2 different results (phantom read). |
| 4 | Serializable | All transactions are completely in isolation fashion, executed serially one after another (lock-based concurrency control). |
In Oracle, Repeated Read level is not supported, but in MySQL default isolation level is repeated Read.
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