The following table lists the values for the RDB$CONSTRAINT_TYPE
    column.

    Value Symbol                      Meaning

    1     RDB$K_CON_CONDITION         Requires conditional expression
                                      constraint.
    2     RDB$K_CON_PRIMARY_KEY       Primary key constraint.
    3     RDB$K_CON_REFERENTIAL       Referential (foreign key)
                                      constraint.
    4     RDB$K_CON_UNIQUE            Unique constraint.
    5                                 Reserved for future use.
    6     RDB$K_CON_NOT_NULL          Not null (missing) constraint.