SQL statements are atomic. That is, if you execute something like this:
UPDATE Cars SET Sold = Sold + 1
Nobody can change the
Sold variable during this statement. It is always incremented by 1, even if somebody else is executing the same statement concurrently.
The problem occurs if you have statements that depend on each other:
a = SELECT Sold FROM Cars; UPDATE Cars SET Sold = a + 1;
Between these queries, another user can change the table Cars and update Sold. To prevent this, wrap it in a transaction:
BEGIN; a = SELECT Sold FROM Cars; UPDATE Cars SET Sold = a + 1; COMMIT;
Transactions are supported by InnoDB, but not by MyISAM.