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<p>^The CREATE TRIGGER statement is used to add triggers to the 
database schema. ^Triggers are database operations 
that are automatically performed when a specified database event
occurs.  </p>

<p>^Each trigger must specify that it will fire for one of
the following operations: [DELETE], [INSERT], [UPDATE].
The trigger fires once for each row that is deleted, inserted,
or updated.  If the "UPDATE OF <yyterm>column-name</yyterm>"
syntax is used, then the trigger will only fire if
<yyterm>column-name</yyterm> appears on the left-hand side of
one of the terms in the SET clause of the [UPDATE] statement.</p>

<p>Due to an historical oversight, columns named in the "UPDATE OF"
clause do not actually have to exist in the table being updated.
Unrecognized column names are silently ignored.
It would be more helpful if SQLite would fail the CREATE TRIGGER
statement if any of the names in the "UPDATE OF" clause are not
columns in the table.  However, as this problem was discovered
many years after SQLite was widely deployed, we have resisted
fixing the problem for fear of breaking legacy applications.</p>

<p>^At this time SQLite supports only FOR EACH ROW triggers, not FOR EACH
STATEMENT triggers. ^Hence explicitly specifying FOR EACH ROW is optional.
^FOR EACH ROW implies that the SQL statements specified in the trigger
may be executed (depending on the WHEN clause) for each database row being
inserted, updated or deleted by the statement causing the trigger to fire.</p>