Constraints

There are three types of constraints: key constraints, foreign key constraints, and check constraints.
· A key constraint is used to prevent duplicate information on a table. This corresponds to first normal form for a relational database (define the key). Key constraints are a prerequisite for foreign key constraints.
· A foreign key constraint (also referred to as referential integrity) defines a relationship between two tables: a dependant and a parent. A foreign key constraint ensures that rows may not be inserted in the dependant table if there isn't a corresponding row in the parent table. It also defines what should be done if a row in the parent table is changed or deleted (more details in a moment).
· A check constraint defines the rules as to which values can be placed in a column.

There are commands available for dealing with constraints on AS/400 (ADDPFCST, CHGPFCST, DSPCPCST, EDTCPCST, RMVPFCST, WRKPFCST), or you can define them in SQL using the CREATE TABLE or ALTER TABLE commands.
But by far the easiest ways of handling constraints is using the Database function in iSeries Navigator.
You can define constraints by selecting Database > System > Schemas > Your Schema > Tables. (In case you are not yet familiar with SQL terminology, a schema is a library and a table is a physical file.) Right-click on a table name and select Definition; the resulting window contains a tab for each type of constraint. On each of these tabs, you have options for Add, Remove, and Definition. The Definition option simply shows you the definition of the constraint; in order to change the definition of a constraint, you must remove it and add it again.