The UPDATE statement updates columns in existing table rows with
new values.  The SET clause indicates which columns to modify
and the values they should be given.  The WHERE clause, if given,
specifies which rows should be updated.  Otherwise, all rows are updated. If
the ORDER BY clause is specified, the rows are updated in the
order that is specified. The LIMIT clause places a limit on the
number of rows that can be updated.

The UPDATE statement supports the following modifiers:


 --- If you specify the LOW_PRIORITY keyword, execution of the
UPDATE is delayed until no other clients are reading from the table.

 --- If you specify the IGNORE keyword, the update statement does not
abort even if errors occur during the update.  Rows for which duplicate-key
conflicts occur are not updated. Rows for which columns are updated to
values that would cause data conversion errors are updated to the closet
valid values instead.
