Constraints

Constraint naming syntax

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

UNIQUE constraint example

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, MySQL 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 Using "CREATE UNIQUE INDEX" to define unique constraints is obsolete (use primary keys or a secondary key instead).

In MySQL, the name of a PRIMARY KEY is PRIMARY.

Unique constraints

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

When using a unique constraint, Informix allows only one row with a NULL value, while MySQL allows several rows with NULL! Using CREATE UNIQUE INDEX is obsolete.

Foreign keys

Both Informix and MySQL support the ON DELETE CASCADE option. In MySQL, foreign key constraints are checked immediately, so NO ACTION and RESTRICT are the same.

Check constraints

Check constraints are not yet supported in MySQL.

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 MySQL.