SQL table definition

Informix®

Informix supports primary key, unique, foreign key, default and check constraints.

The constraint naming syntax is different in Informix and most other databases: Informix expects the constraint name after the constraint definition:

CREATE TABLE emp (
  ...
  emp_code CHAR(10) UNIQUE CONSTRAINT pk_emp,
  ...
)

While other SQL database brands require to specify the constraint name before the constraint definition:

CREATE TABLE emp (
   ... 
   emp_code CHAR(10) CONSTRAINT pk_emp UNIQUE, 
   ...
)

PostgreSQL

PostgreSQL support primary key, unique, foreign key, default and check constraints.

Constraint naming syntax

The constraint naming clause must be placed before the constraint specification.

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 PostgreSQL.

Unique constraints

When using a unique constraint, Informix allows only one row with a NULL value, while PostgreSQL allows several rows with NULL!

Consider using NOT NULL constraint with UNIQUE.