Constraints

Five types of constraints can be specified.They are:

Primary key (PRIMARY KEY)
Unique key (UNIQUE)
Foreign key (REFERENCES)
CHECK constraint (CHECK)
NOT NULL constraint (NOT NULL)

Constraints can be specified at column level (“column constraints”) or at table level (“table constraints”).Table-level constraints are required when keys (unique constraint, primary key, foreign key) consist of multiple columns and when a CHECK constraint involves other columns in the row besides the column being defined.The NOT NULL constraint can only be specified as a column constraint.Syntax for some types of constraint may differ slightly according to whether the constraint is defined at the column or table level.

A column-level constraint is specified during a column definition, after all column attributes except COLLATION are specified, and can involve only the column specified in that definition
A table-level constraints can only be specified after the definitions of the columns used in the constraint.
Table-level constraints are a more flexible way to set constraints, since they can cater for constraints involving multiple columns
You can mix column-level and table-level constraints in the same CREATE TABLE statement

The system automatically creates the corresponding index for a primary key (PRIMARY KEY), a unique key (UNIQUE), and a foreign key (REFERENCES for a column-level constraint, FOREIGN KEY REFERENCES for table-level).

Names for Constraints and Their Indexes

Constraints and their indexes are named automatically if no name was specified using the CONSTRAINT clause:

The constraint name has the form INTEG_n, where n represents one or more digits
The index name has the form RDB$PRIMARYn (for a primary key index), RDB$FOREIGNn (for a foreign key index) or RDB$n (for a unique key index).