Transactions
Sometimes you want to insert data in different tables, but you want to
insert them as a group of data. In other words, the integrity of
the group insert is dependent on each individual insert working.
If any of them fail, you want to erase what you did and start over from
the beginning. This is a Transaction.
- You can make a transaction with the assistance of COMMIT and ROLLBACK statements.
- These are database terms. A COMMIT indicates that you are pleased with the transaction and wish to make it final. A ROLLBACK states that something went wrong, and you wish to go back to the initial state.
- By default, inserts and updates you make are auto-committed; that is, they are committed when executed.
- But, if you want to run these as a transaction, you need to turn autocommit off and send the COMMIT message itself.
- If something goes wrong before the COMMIT message is reached, an exeption will usually be thrown. A SQLException, for instance. Then, in the exception handler, you can issue a ROLLBACK statement.
- From this we learn two things: 1) try-catch blocks do have significant value, and 2) Wal-Mart didn't invent the word ROLLBACK.