Constraints Review

What is a constraint? Unique – forbids duplicate values Referencial – Foreign key Check Constraint – sets restrictions on data added to a specific table

SQL vs ADDPFCST

Database Triggers

Trigger? A condition that causes some procedure to be executed. On the Iseries, a program that is called when a program tries to insert, update or delete a database record A trigger can be more complex than a check constraint

Writing Trigger Programs Just as you do any HLL program AddPFTrg (Add Physical File Trigger) to associate the program with the database file.

6 Trigger Conditions Before Insert Before Update Before Delete After Insert After Update After Delete

Advantages Be sure that the actions coded in the trigger program are executed irregardless of the application that is trying to modify the database

Disadvantages You must be aware that triggers exist

RmvPfTrg Removes a trigger program from a database

Coding a Trigger Program Can be coded in any iSeries HLL Must have 2 parameters that conform to IBM-defined interface for trigger programs –Trigger buffer Before and after image of the affected row See Fig 16.1 in text book –Trigger length Length of the buffer

Trigger Program Trigger Programs must cause an exception message. Best way to do this is to send a program message to the calling program. This causes the application to crash and the database update is halted.

Vendor Master CREATE TABLE VENDOR (VENID DEC (7, 0) NOT NULL WITH DEFAULT, NAME CHAR (30 ) NOT NULL WITH DEFAULT, MINORD DEC (7, 0) NOT NULL WITH DEFAULT, CONSTRAINT VENDORPK PRIMARY KEY (VENID))

Write a Trigger Program Minimum order must be greater than

Student Master CREATE TABLE STUDENT (STUDID NUMERIC (9, 0), SNAME CHAR (30 ), PROV CHAR (15), ACTBAL NUMERIC(11, 2))

Write a trigger program that will not allow records to be updated where the PROV = ON and the ACTBAL < 100