Step 2: Selecting Records

To update customer records, you first must identify and retrieve the records. You use the NEXT command to select the records and copy the relevant information (customer ID, name, address, and phone number) into the CustInfo stack.

Methods: NEXT and MATCH

To read data from a data source, you first must select the record where the data resides. In MODIFY, there were two ways of doing this:

By field value—one segment instance at a time, one record at a time—using MATCH.
Sequentially—one segment instance at a time, one record at a time—using NEXT.

For example, before MODIFY displays a record in a CRTFORM or writes it to the SPA, you must select it using MATCH or NEXT. You also must identify the record before MODIFY includes, updates, or deletes it. Thirdly, you can only identify one segment instance, in one record, at a time.

Maintain releases you from these restrictions. In Maintain, there are five ways of selecting records:

By field value—for an entire set of records—using NEXT. The WHERE phrase enables you to select by value, and the FOR ALL phrase selects the entire set of records that satisfies the WHERE selection condition. The basic syntax for this is:
```
FOR ALL NEXT fields INTO stack WHERE selection_condition;
```
By field value—for a sequence (subset) of records—using NEXT. This method is similar to selecting by field value for an entire set except that it employs the FOR n phrase, selecting—at the current position in the data source—the first n records that satisfy the WHERE condition. The basic syntax for this is:
```
FOR n NEXT fields INTO stack WHERE selection_condition;
```
By field value—one segment at a time, one record at a time—using MATCH. This method works very much like MATCH in MODIFY, except that the values on which you are matching can be taken from a source stack, and the fields that you retrieve can be placed into a destination stack. The basic syntax for this is:
```
MATCH fields [FROM stack] [INTO stack];
```
Sequentially—for a sequence (subset) of records—using NEXT. This method employs the FOR n phrase to select the next n records. The basic syntax for this is:
```
FOR n NEXT fields INTO stack;
```
Sequentially—one segment instance at a time, one record at a time—using NEXT. This method works very much like NEXT in MODIFY, except that the fields which you retrieve can be placed into a destination stack. The basic syntax for this is:
```
NEXT fields [INTO stack];
```

Specifying Data Source Position With the REPOSITION Command

Each time you issue a NEXT command, Maintain begins searching for records from the current position in the data source. For example, if your first data source operation retrieved a set of records:

FOR ALL NEXT CustID INTO SmokeStack
WHERE ProdName EQ 'VCR DUST COVER';

Maintain has searched sequentially through the entire data source. The current position marker now points to the end of the data source. If you issue another NEXT command:

FOR ALL NEXT LastName FirstName INTO CandyStack
WHERE ProdName EQ 'CANDY';

Maintain searches from the current position to the end of the data source. Because the current position is the end of the data source, no records are found.

When you want a NEXT command to search through the entire data source (often the case when you wish to retrieve a set of records), you must first issue the REPOSITION command to move the current position marker to the beginning of the data source.

For example, the following REPOSITION command specifies the CustID field—which is in the root segment—and so moves the current position marker for the root segment chain and all of its descendant chains back to the beginning of the chain (in effect, back to the beginning of the data source):

REPOSITION CustID;
FOR ALL NEXT LastName FirstName INTO CandyStack
WHERE ProdName EQ 'CANDY';

REPOSITION is similar to the MODIFY command of the same name.

Solution

You add a NEXT command to retrieve the desired records. Each part of the command plays a different role:

FOR ALL selects a set of records.
WHERE ExpDate GE 920601 AND ExpDate LE 920621 restricts the set to the customers whose memberships are expiring during the first three weeks of June.
INTO CustInfo copies all the fields from the specified segments—in this case, the root segment—into the stack named CustInfo.

The procedure now looks like this:

MAINTAIN FILE VideoTrk 
FOR ALL NEXT CustID INTO CustInfo
WHERE ExpDate GE 920601 AND ExpDate LE 920621; 
END

Try it now: If you have created your own set of application files, enter these additional commands into your FOCEXEC file.