Constraints

Constraint naming syntax

Both Informix^® and DB2^® support primary key, unique, foreign key, default and check constraints. But the constraint naming syntax is different: DB2 expects the "CONSTRAINT" keyword before the constraint specification, and Informix expects it after .

UNIQUE constraint example:

Table 1. UNIQUE constraint example (Informix vs IBM^® DB2)
Informix	IBM DB2
CREATE TABLE emp ( ... emp_code CHAR(10) UNIQUE CONSTRAINT pk_emp,	CREATE TABLE emp ( ... emp_code CHAR(10) CONSTRAINT pk_emp UNIQUE, ...

Primary keys

Like Informix, DB2 creates an index to enforce PRIMARY KEY constraints (some RDBMS do not create indexes for constraints). Using "CREATE UNIQUE INDEX" to define unique constraints is obsolete (use primary keys or a secondary key instead).

Note: DB2 primary key constraints do not allow NULLs; make sure your tables do not contain NULLs in the primary key columns.

Unique constraints

Like Informix, DB2 creates an index to enforce UNIQUE constraints (some RDBMS do not create indexes for constraints).

Note: DB2 unique constraints do not allow NULLs; make sure your tables do not contain NULLs in the unique columns.

Foreign keys

Both Informix and DB2 support the ON DELETE CASCADE option.

Check constraints

The check condition may be any valid expression that can be evaluated to TRUE or FALSE,including functions and literals. You must verify that the expression is not Informix-specific.

Null constraints

Informix and DB2 support NOT NULL constraints, but Informix does not allow you to give a name to "NOT NULL" constraints.

Solution

Constraint naming syntax: The database interface does not convert constraint naming expressions when creating tables from BDL programs. Review the database creation scripts to adapt the constraint naming clauses for DB2.