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Triggers in Informix SPL
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Informix also supports triggers through a CREATE TRIGGER statement. As in the TransactSQL dialect, the beginning of the CREATE TRIGGER statement defines the trigger name, the table on which the trigger is being defined, and the triggering actions. Here are statement fragments that show the syntax:
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create trigger new_sls insert on salesreps . . . create trigger del_cus_chk delete on customers . . . create trigger ord_upd update on orders . . . create trigger sls_upd update of quota, sales on salesreps . . .
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The last example is a trigger that fires only when two specific columns of the SALESREPS table are updated.
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Informix allows you to specify that a trigger should operate at three distinct times during the processing of a triggered change to the target table: BEFORE The trigger fires before any changes take place. No rows of the target table have yet been modified. AFTER The trigger fires after all changes take place. All affected rows of the target table have been modified. FOR EACH ROW The trigger fires repeatedly, once as each row affected by the change is being modified. Both the old and new data values for the row are available to the trigger. An individual trigger definition can specify actions to be taken at one or more of these steps. For example, a trigger could execute a stored procedure to calculate the sum of all orders BEFORE an update, monitor updates to each ORDERS row as they occur with a second action, and then calculate the revised order total AFTER the update with a call to another stored procedure. Here is a trigger definition that does all of this:
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create trigger upd_ord update of amount on orders referencing old as pre new as post /* Calculate order total before changes */ before (execute procedure add_orders() into old_total;) /* Capture order increases and decreases */ for each row when (post.amount < pre.amount) /* Write decrease data into table */ (insert into ord_less values (pre.cust, pre.order_date, pre.amount, post.amount);) when (post.amount > pre.amount) /* Write increase data into table */ (insert into ord_more values (pre.cust, pre.order_date, pre.amount, post.amount);) /* After changes, recalculate total */ after (execute procedure add_orders() into new_total;)
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The BEFORE clause in this trigger specifies that a stored procedure named ADD_ORDERS is to be called before any UPDATE statement processing occurs. Presumably, this procedure calculates the total orders and returns the total value into the local variable OLD_TOTAL. Similarly, the AFTER clause specifies that a stored procedure (in this case, the same one) is to be called after all UPDATE statement processing is complete. This time, the total orders amount is placed into a different local variable, NEW_TOTAL.
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20:
Database Processing and Stored Procedural SQL
The FOR EACH ROW clause specifies the action to be taken as each affected row is updated. In this case, the requested action is an INSERT into one of two order-tracking tables, depending on whether the order amount is being increased or decreased. These tracking tables contain the customer number, date, and both the old and new order amounts. To obtain the required values, the trigger must be able to refer to both the old (prechange) and the new (postchange) values of each row. The REFERENCING clause provides names by which these two states of the row currently being modified in the ORDERS table can be used. In this example, the prechange values of the columns are available through the column name qualifier PRE, and the postchange values are available through the column name qualifier POST. These are not special names; any names can be used. Informix is more limited than some other DBMS brands in the actions that can be specified within the trigger definition itself. These statements are available: INSERT DELETE UPDATE EXECUTE PROCEDURE In practice, the last option provides quite a bit of flexibility. The called procedure can perform almost any processing that could be done inline within the trigger body itself.
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