Creates a trigger for the specified relation.  A trigger is a
 mechanism which associates a set of rules with an update operation.

 Example:

 RDO> DEFINE TRIGGER EMPLOYEE_ID_CASCADE_DELETE  <--- trigger name
 cont>        BEFORE ERASE                       <--- action time
 cont>        FOR E IN EMPLOYEES            <--- subject relation
 cont>            EXECUTE
 cont>                FOR D IN DEGREES WITH <--- triggered action rse
 cont>                  D.EMPLOYEE_ID = E.EMPLOYEE_ID
 cont>                  ERASE D             <--- triggered action
 cont>                END_FOR;
 cont>            FOR EACH RECORD.          <--- frequency clause

Additional information available:

MoreFormatExample

 Each trigger is associated with a single subject relation, will be
 evaluated at a specific time for a particular type of update on that
 relation, and specifies a series of 'triggered' actions.  Each
 triggered action consists of an optional condition and one or more
 statements to be evaluated either once only or for each record of the
 relation being updated.

 You can use triggers with relation-specific constraints to preserve
 database integrity.

 You can define a trigger only after you have invoked the database.
 See the DEFINE_RELATION statement.

 Defining a trigger requires READ and DEFINE access to the subject
 relation.  If any triggered statement specifies some form of update
 operation, then READ, CONTROL and appropriate update access (ERASE,
 MODIFY, or WRITE) to the relations specified by the triggered action
 statements is also required.

 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 TRIGGER statement.

 DEFINE TRIGGER ─> name ┬──────────────>─────────────┬┬─> 
B
E
F
O
R
E ┬┐
                        └> 
D
E
S
C
R
I
P
T
I
O
N typebox (I)typebox (S) typebox (/)typebox (*) text */ ┘└─> 
A
F
T
E
R ─┘│
 ┌─────────────────────────────<──────────────────────────────────┘
 └┬> 
S
T
O
R
E ───────────────────>──────────────────────────────────┬┐
  ├> 
E
R
A
S
E ───────────────────>──────────────────────────────────┤│
  └> 
M
O
D
I
F
Y ┬──────────>──────────┬┬──────────────>────────────┬─┘│
            └> 
O
F ─┬> field-name ┬┘└> context-variable-clause ─┘  │
                   └───── , <────┘                                │
 ┌──────────────────────────────────<─────────────────────────────┘
 └> 
F
O
R context-var IN relation-name ┬> triggered-action-clause ┬> .
                                     └──────────────<───────────┘

Additional information available:

nametextfield-namecontext-variable-clausecontext-varrelation-name
triggered-action-clause

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

 Use a name that is unique among all trigger names.

 A text string that adds a comment to the trigger definition.

 The name of a field within the specified relation to be checked for
 modification.

 context-variable-clause =
 ──┬────────────────────────────────┬─>
   ├─> 
O
L
D typebox (C)typebox (O)typebox (N)typebox (T)typebox (E)Xtypebox (T) old-context-var ─┤
   └─> 
N
E
W typebox (C)typebox (O)typebox (N)typebox (T)typebox (E)Xtypebox (T) new-context-var ─┘

An optional clause that can contain aliases for old and new states of
 the context variable of the triggering statement.

Additional information available:

old-context-varnew-context-var

 A temporary name used to refer to the record values as they existed
 before a modify operation occurred.  You use an old-context-var only
 when the trigger occurs after a modify operation, and must be a
 different name than the one given for context_var.

 A temporary name used to reference the new record values about to be
 applied by modify operation.  You use a new-context-var only when the
 trigger occurs before a modify operation, and must be a different
 name than the one given for context-var.

 A temporary name defining the relation on which the trigger is
 defined.  The context variable name is an alias for the record stream
 context of the triggering statement which has caused this trigger to
 be executed.

 The name of the relation for which this trigger is defined (subject
 relation).

 triggered-action-clause =
 ─┬───────────────────────────┬─┐
  └─> 
W
I
T
H conditional-expr ──┘ │
  ┌─────────────────────────────┘
  └─> 
E
X
E
C
U
T
E ─┬> triggered-statement ─┬──┬───────────────────┬─>
               └─────────── typebox (;) ────<────┘  └─> 
F
O
R 
E
A
C
H 
R
E
C
O
R
D ┘

 triggered-statement =
 ─┬─> store-statement ───────────────────┬─>
  ├─> 
F
O
R rse store-statement ENDFOR  ──┤
  ├─> 
F
O
R rse erase-statement ENDFOR  ──┤
  ├─> 
F
O
R rse modify-statement ENDFOR ──┤
  └─> 
E
R
R
O
R ─────────────────────────────┘

 A triggered action clause consists of a conditional expression
 (optional), one or more triggered statements, and a frequency clause
 (FOR EACH RECORD).  The EXECUTE keyword precedes the triggered action
 clause.

Additional information available:

WITHFOR_EACH_RECORD

rsestore-statementerase-statementmodify-statement

 A record selection expression that defines which records of which
 relations will be affected by the triggered update action.  This rse
 cannot refer to any host variables.

 A STORE statement to be initiated by the trigger.

 An ERASE statement to be initiated by the trigger.

 A MODIFY statement to be initiated by the trigger.

 A conditional expression that describes the optional condition that
 must be satisfied before the associated triggered statements are
 executed.  This expression cannot refer to any host variables.  For
 more information on Rdb/VMS conditional expressions, ask for HELP on
 the top level topic Cond_expr.

 A frequency clause, FOR EACH RECORD (no underscores), determines
 whether an action is evaluated once per triggering statement, or for
 each record of the subject relation updated by the triggering
 statement.  If the FOR EACH RECORD clause is not specified, the
 triggered action is evaluated only once, and record values are not
 available to the triggered action.

 This example defines a trigger that performs a cascading delete
 triggered by the deletion of an employee record.  Such a trigger can
 be used to maintain referential integrity among the EMPLOYEES,
 JOB_HISTORY, RESUMES, and SALARY_HISTORY relations.
 Each associated employee record (from the relations which have
 foreign keys referring to the primary key in the EMPLOYEES relation)
 is deleted.

 RDO> DEFINE TRIGGER EMPLOYEE_ID_CASCADE_DELETE
 cont>        BEFORE ERASE
 cont>        FOR E IN EMPLOYEES
 cont>            EXECUTE
 cont>                FOR D IN DEGREES WITH
 cont>                  D.EMPLOYEE_ID = E.EMPLOYEE_ID
 cont>                  ERASE D
 cont>                END_FOR;
 cont>                FOR JH IN JOB_HISTORY WITH
 cont>                  JH.EMPLOYEE_ID = E.EMPLOYEE_ID
 cont>                  ERASE JH
 cont>                END_FOR;
 cont>                FOR R IN RESUMES WITH
 cont>                  R.EMPLOYEE_ID = E.EMPLOYEE_ID
 cont>                  ERASE R
 cont>                END_FOR;
 cont>                FOR SH IN SALARY_HISTORY WITH
 cont>                  SH.EMPLOYEE_ID = E.EMPLOYEE_ID
 cont>                  ERASE SH
 cont>                END_FOR
 cont>            FOR EACH RECORD.