Creates a constraint for a relation or relations.  A constraint
 defines a set of conditions that restrict the values stored in
 relations.  When you store and modify field values, the constraint
 checks the validity of the values and generates an error message if
 the constraint is violated.

 Example:

    RDO>  DEFINE CONSTRAINT EMPLOYEE_ID_REQUIRED
    cont>  FOR E IN EMPLOYEES
    cont>  REQUIRE E.EMPLOYEE_ID NOT MISSING.

Additional information available:

MoreFormatExample

 To define a constraint, you must have Rdb/VMS READ access to the
 database and Rdb/VMS READ and DEFINE access to all relations to which
 the constraint refers.

 When the DEFINE CONSTRAINT statement executes, the constraint
 definition is added to the physical database.  If you have invoked
 the database using the PATHNAME argument, the constraint definition
 is also stored in the data dictionary.

 The DEFINE CONSTRAINT statement includes a record selection
 expression.  Therefore, it is more flexible for checking values on
 input than the VALID IF clause.  For example:

  o  VALID IF must be part of a DEFINE FIELD statement.  Therefore, a
     VALID IF criterion applies to all the fields that use the
     definition.  You can define a constraint that refers to only one
     of several relations that use a global field definition.

  o  VALID IF refers only to a range of literal values.  You cannot
     use VALID IF to check a value against values of fields stored in
     the database.  With DEFINE CONSTRAINT, you can check values from
     one relation against other database values, either in the same or
     another relation.

  o  You can use DEFINE CONSTRAINT to check for such conditions as
     existence, uniqueness, and nonexistence.


 Rdb/VMS evaluates constraints by validating existing data against the
 RSE specified by the constraint.  If there is no existing data for
 Rdb/VMS to validate, the constraint will be defined without being
 evaluated.

 Rdb/VMS evaluates constraints at definition time; therefore, you
 cannot define a new constraint that violates an existing constraint.

 You can also specify that a constraint be checked when the STORE or
 MODIFY statement executes (CHECK ON UPDATE) or when the COMMIT
 statement executes (CHECK ON COMMIT).  In this way, you can include
 interlocking constraints.  For example, you might define two
 constraints to ensure that a department cannot exist without
 employees and an employee must belong to a department.  However, if
 these two constraints were checked on UPDATE, there would be no way
 to create a new department.  Instead, they should be checked on
 COMMIT.  You can then create a department and give it members inside
 a single transaction, and check the constraints when the COMMIT
 statement ends the transaction.

 You can define a constraint only after you have invoked the database.
 See the INVOKE statement.

 You must execute this statement in a read/write transaction.  If
 there is no active transaction and you issue this statement, Rdb/VMS
 starts a read/write transaction implicitly.

 Other users are allowed to be attached to the database when you issue
 the DEFINE CONSTRAINT statement.

 DEFINE CONSTRAINT ─────> name ────┐
       ┌───────────────────────────┘
       └────┬─────────────────────────────────┬────┐
            └─> 
D
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C
R
I
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T
I
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N typebox (I)typebox (S) typebox (/)typebox (*) text */ ────┘    │
       ┌───────────────────────────────────────────┘
       └─── 
F
O
R ───> rse ───┐
       ┌────────────────────┘
       └──> 
R
E
Q
U
I
R
E ───> conditional-expr ──────┐
       ┌────────────────────────────────────────┘
       └────┬────────────────────────────────┬────> .
            └─> 
C
H
E
C
K typebox (O)typebox (N) ──┬───> 
C
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                           └───> 
U
P
D
A
T
E ──┘

Additional information available:

CHECK

nametextrseconditional-expression

 The name of the constraint that you can refer to in other statements.
 When choosing a name, follow these rules:

  o  Use a name that is unique among all constraint names.
  o  Use any valid VMS name.  However, the name cannot end in a dollar
     sign ($) or underscore (_).
  o  Do not use any Rdb/VMS reserved words (see the VAX Rdb/VMS RDO
     Reference Manual appendix for reserved words).

 A text string that adds a comment to the field definition.  You can
 apply the description to the entire relation definition using the
 DESCRIPTION keyword.  You can also apply a separate description to
 each field using the text markers before the DEFINE, CHANGE, or
 DELETE clause.

 A record selection expression that defines which records of which
 relations the constraint applies to.

 A conditional expression that describes the constraint to be placed
 on the records and relations defined by the RSE.  For more
 information on Rdb/VMS conditional expressions, ask for HELP on
 Cond_expr.

 Specifies whether the constraint is to be evaluated when you issue a
 statement such as STORE or MODIFY that updates the relation (UPDATE),
 or when you issue the COMMIT statement to write the change to the
 database (COMMIT).  You can override this qualifier with the
 EVALUATING clause of the START_TRANSACTION statement.  The default is
 UPDATE.

 Check for the existence of a field value in another relation:

 DEFINE CONSTRAINT DEPT_CODE_EXISTS
   FOR JH IN JOB_HISTORY
     REQUIRE ANY D IN DEPARTMENTS WITH
     D.DEPARTMENT_CODE = JH.DEPARTMENT_CODE.

 The ANY operator is equivalent to saying "there exists".  This
 constraint therefore means "For every record in JOB_HISTORY, require
 that there exists a record in DEPARTMENTS where the DEPARTMENT_CODE
 values match."