Assertions and triggers1

2 Constraints Attribute-based CHECK constraints create table … ( postcode number(4) check (postcode > 0) ); Checked at update to the table. Tuple-based CHECK constraints create table … ( … check (gender = 'M' or name = 'Mary') ); Checked at update to the table. Schema-level ASSERTIONS –Checked at any update to the tables in the assertion. –Not available in Oracle.

Assertions and triggers3 Assertions Syntax –create assertion name check (condition) Has the ability to refer to all attributes in the database. Examples from Garcia-Molina –Fig fig. 7.7, page 338 –Example 7.14, page 339

Assertions and triggers4 CHECK doesn't give guarantees, 339 CHECK conditions are checked when the attribute / row is updates. Condition is not checked if other data are changed –CHECK conditions with sub-queries are not guaranteed to hold –Examples Fig. 7.7, page 338 Example, page 339

Assertions and triggers5 Assertions vs. triggers Assertions must be checked at any change to the mentioned (in the assertion declaration) relations. –That takes a lot of time! –Oracle doesn't implement assertions!! Triggers are executed at certain events specified by the database user –Not on every update. –That takes less time!! –Oracle implements triggers!!

Assertions and triggers6 Triggers, syntax, 328 create trigger triggerName … –Shah page 328 Event based execution –BEFORE the event (insert / update) –AFTER the event –INSTEAD OF the event (works on views only)

Assertions and triggers7 BEFORE triggers, 330 Executed BEFORE the insert / update is executed by the DBMS –Example: changeNullTrigger.sql –Better solution: Default value on the column Shah page 330 –Hides the use of sequences (Oracle feature) from the application –Inserts the current date –You can refer to the pseudo variable :NEW

Assertions and triggers8 AFTER trigger, 331 Executed AFTER the insert / update is executed by the DBMS –You can refer to the pseudo variables :NEW and :OLD –Examples: Shah fig , page 332 Employee_adu_triggerAfter.sql Used to –Check the certain conditions are still true after the insert / update

Assertions and triggers9 Instead-of triggers, 333 Not part of the SQL3 standard, but common i commercial DBMS's Ordinary triggers –Before / after the update –for updating base tables. Instead-of triggers –executed instead of the update –for updating views which would otherwise be non-updatable Example Shah fig , page , page 334 Full_employee_insteadOfTrigger.sql