4D Doc Center : SET QUERY DESTINATION

4D v13.4

SET QUERY DESTINATION


SET QUERY DESTINATION ( destinationType {; destinationObject {; destinationPtr}} )

Parameter	Type	Description
destinationType	Longint	0 = current selection, 1 = set, 2 = named selection, 3 = variable
destinationObject	String, Variable	Name of the set, or Name of the named selection, or Variable
destinationPtr	Pointer	Pointer to local variable if destinationType=3

Description

SET QUERY DESTINATION enables you to tell 4D where to put the result of any subsequent query for the current process.

You specify the type of the destination in the parameter destinationType. 4D provides the following predefined constants, found in the "Queries" theme:

Constant	Type	Value
Into current selection	Longint	0
Into named selection	Longint	2
Into set	Longint	1
Into variable	Longint	3

You specify the destination of the query itself in the optional destinationObject parameter according to the following table:

destinationType	destinationObject
parameter	parameter
0 (current selection)	You omit the parameter
1 (set)	You pass the name of a set (existing or to be created)
2 (named selection)	You pass the name of a named selection (existing or to be created)
3 (variable)	You pass a numeric variable (existing or to be created) or an empty string "" to use the destinationPtr parameter

With:

SET QUERY DESTINATION(Into current selection)

The records found by any subsequent query will end up in a new current selection for the table involved by the query.

With:

SET QUERY DESTINATION(Into set;"mySet")

The records found by any subsequent query will end up in the set "mySet". The current selection and the current record for the table involved by the query are left unchanged.

Note: In client/server, you cannot use local/client sets (name preceeded by $ symbol) as a query destination. This type of set is created on client machines when queries are executed on the server. For more information on these types of sets, refer to the Sets section.

With:

SET QUERY DESTINATION(Into named selection;"myNamedSel")

The records found by any subsequent query will end up in the named selection "myNamedSel". The current selection and the current record for the table involved by the query are left unchanged.

Notes:

If the named selection does not exist beforehand, it will be created automatically at the end of the query.
This command manages named selections like the CUT NAMED SELECTION command: only references are kept. Once the named selection is used, it no longer exists.

With:

SET QUERY DESTINATION(Into variable;$vlResult)

Or:

SET QUERY DESTINATION(Into variable;"";->$vlResult)

The number of records found by any subsequent query will end up in the variable $vlResult. The current selection and the current record for the table involved by the query are left unchanged.

Warning: SET QUERY DESTINATION affects all subsequent queries made within the current process. REMEMBER to always counterbalance a call to SET QUERY DESTINATION (where destinationType#0) with a call to SET QUERY DESTINATION(0) in order to restore normal query mode.

SET QUERY DESTINATION changes the behavior of the query commands only:

On the other hand, SET QUERY DESTINATION does not affect other commands that may change the current selection of a table such as ALL RECORDS, RELATE MANY and so on.

Example 1

You create a form that will display the records from a [Phone Book] table. You create a Tab Control named asRolodex (with the 26 letters of the alphabet) and a subform displaying the [Phone Book] records. Choosing one Tab from the Tab Control displays the records whose names start with the corresponding letter.

In your application, the [Phone Book] table contains a set of quite static data, so you do not want to (or need to) perform a query each time you select a Tab. In this way, you can save precious database engine time.

To do so, you can redirect your queries into named selections that you reuse as needed. You write the object method of the Tab Control asRolodex as follows:

  ` asRolodex object method

 Case of

    :(Form event=On Load)

  ` Before the form appears on the screen,

  ` initialize the rolodex and an array of Booleans that

  ` will tell us if a query for the corresponding letter

  ` has been performed or not

       ARRAY STRING(1;asRolodex;26)

       ARRAY BOOLEAN(abQueryDone;26)

       For($vlElem;1;26)

          asRolodex{$vlElem}:=Char(64+$vlElem)

          abQueryDone{$vlElem}:=False

       End for

 

    :(Form event=On Clicked)

  ` When a click on the Tab control occurs, check whether the corresponding query

  ` has been performed or not

       If(Not(abQueryDone{asRolodex}))

  ` If not, redirect the next query(ies) toward a named selection

          SET QUERY DESTINATION(Into named selection;"temp")

  ` Perform the query

          QUERY([Phone Book];[Phone Book]Last name=asRolodex{asRolodex}+"@")

  ` Restore normal query mode

          SET QUERY DESTINATION(Into current selection)

  ` Use the records found

          USE NAMED SELECTION("temp")

          COPY NAMED SELECTION([Phone book];"Rolodex+asRolodex{asRolodex})

  ` Next time we choose that letter, we won't perform the query again

          abQueryDone{asRolodex}:=True

       Else

  ` Use the existing named selection for displaying the records corresponding to the chosen letter

          USE NAMED SELECTION("Rolodex"+asRolodex{asRolodex}

       End if

 

    :(Form event=On Unload)

  ` After the form disappeared from the screen

  ` Clear the named selections we created

       For($vlElem;1;26)

          If(abQueryDone{$vlElem})

             CLEAR NAMED SELECTION("Rolodex"+asRolodex{$vlElem})

          End if

       End for

  ` Clear the two arrays we no longer need

       CLEAR VARIABLE(asRolodex)

       CLEAR VARIABLE(abQueryDone)

 End case

Example 2

The Unique values project method in this example allows you to verify the uniqueness of the values for any number of fields in a table. The current record can be an existing or a newly created record.

  ` Unique values project method

  ` Unique values ( Pointer ; Pointer { ; Pointer... } ) -> Boolean

  ` Unique values ( ->Table ; ->Field { ; ->Field2... } ) -> Yes or No

 

 C_BOOLEAN($0;$2)

 C_POINTER(${1})

 C_LONGINT($vlField;$vlNbFields;$vlFound;$vlCurrentRecord)

 $vlNbFields:=Count parameters-1

 $vlCurrentRecord:=Record number($1->)

 If($vlNbFields>0)

    If($vlCurrentRecord#-1)

       If($vlCurrentRecord<0)

  ` The current record is an unsaved new record (record number is -3);

  ` therefore we can stop the query as soon as at least one record is found

          SET QUERY LIMIT(1)

       Else

  ` The current record is an existing record;

  ` therefore we can stop the query as soon as at least two records are found

          SET QUERY LIMIT(2)

       End if

  ` The query will return its result in $vlFound

  ` without changing the current record nor the current selection

       SET QUERY DESTINATION(Into variable;$vlFound)

  ` Make the query according to the number of fields that are specified

       Case of

          :($vlNbFields=1)

             QUERY($1->;$2->=$2->)

          :($vlNbFields=2)

             QUERY($1->;$2->=$2->;*)

             QUERY($1->;&;$3->=$3->)

          Else

             QUERY($1->;$2->=$2->;*)

             For($vlField;2;$vlNbFields-1)

                QUERY($1->;&;${1+$vlField}->=${1+$vlField}->;*)

             End for

             QUERY($1->;&;${1+$vlNbFields}->=${1+$vlNbFields}->)

       End case

       SET QUERY DESTINATION(Into current selection) ` Restore normal query mode

       SET QUERY LIMIT(0) ` No longer limit queries

  ` Process query result

       Case of

          :($vlFound=0)

             $0:=True ` No duplicated values

          :($vlFound=1)

             If($vlCurrentRecord<0)

                $0:=False ` Found an existing record with the same values as the unsaved new record

             Else

                $0:=True ` No duplicated values; just found the very same record

             End if

          :($vlFound=2)

             $0:=False ` Whatever the case is, the values are duplicated

       End case

    Else

       If(◊DebugOn) ` Does not make sense; signal it if development version

          TRACE ` WARNING! Unique values is called with NO current record

       End if

       $0:=False ` Can't guarantee the result

    End if

 Else

    If(◊DebugOn) ` Does not make sense; signal it if development version

       TRACE ` WARNING! Unique values is called with NO query condition

    End if

    $0:=False ` Can't guarantee the result

 End if

After this project method is implemented in your application, you can write:

  ` ...

 If(Unique values(->[Contacts];->[Contacts]Company);->[Contacts]Last name;->[Contacts]First name)

  ` Do appropriate actions for that record which has unique values

 Else

    ALERT("There is already a Contact with this name for this Company.")

 End if

  ` ...