Step-by-Step Programming with Base SAS 9.4, Second Edition, 2nd Edition

The following output displays the modified INVENTORY_TOOL data set:

Figure 21.2 The INVENTORY_TOOL Data Set with Updated Prices

Modifying a Master Data Set with Observations

from a Transaction Data Set

Understanding the MODIFY Statement

The MODIFY statement replaces data in a master data set with data from a transaction

data set, and makes the changes in the original master data set. You can use a BY

statement to match observations from the transaction data set with observations in the

master data set. The syntax for using the MODIFY statement and the BY statement

follows:

MODIFY master-SAS-data-set transaction-SAS-data-set;

BY by-variable;

Master-SAS-data-set specifies the SAS data set that you want to modify. Transaction-

SAS-data-set specifies the SAS data set that provides the values for updating the master

data set. By-variable specifies one or more variables by which you identify

corresponding observations.

When you use a BY statement with the MODIFY statement, the DATA step uses

dynamic WHERE processing to find observations in the master data set. Neither the

master data set nor the transaction data set needs to be sorted. However, for large data

sets, sorting the data before you modify it can enhance performance significantly.

338 Chapter 21 • Modifying SAS Data Sets

Adding New Observations to the Master Data Set

You can use the MODIFY statement to add observations to an existing master data set. If

the transaction data set contains an observation that does not match an observation in the

master data set, then SAS enables you to write a new observation to the master data set if

you use an explicit OUTPUT statement in your program. When you specify an explicit

OUTPUT statement, you must also specify a REPLACE statement if you want to replace

observations in place. All new observations append to the end of the master data set.

Checking for Program Errors

You can use the _IORC_ automatic variable for error checking in your DATA step

program. The _IORC_ automatic variable contains the return code for each I/O operation

that the MODIFY statement attempts to perform.

The best way to test the values of _IORC_ is with the mnemonic codes that are provided

by the SYSRC autocall macro. Each mnemonic code describes one condition. The

mnemonics provide an easy method for testing problems in a DATA step program. The

following is a partial list of codes:

_DSENMR

specifies that the transaction data set observation does not exist in the master data set

(used only with MODIFY and BY statements). If consecutive observations with

different BY values do not find a match in the master data set, then both of them

return _DSENMR.

_DSEMTR

specifies that multiple transaction data set observations with a given BY value do not

exist in the master data set (used only with MODIFY and BY statements). If

consecutive observations with the same BY values do not find a match in the master

data set, then the first observation returns _DSENMR and the subsequent

observations return _DSEMTR.

_DSENOM

specifies that no matching observation was found.

_SOK

specifies that the observation was located in the master data set and indicates that the

MODIFY statement executed successfully.

The Program

The program in this section updates values in a master data set with values from a

transaction data set. If a transaction does not exist in the master data set, then the

program adds the transaction to the master data set.

In this example, a warehouse received a shipment of new items, and the

INVENTORY_TOOL master data set must be modified to reflect the changes. The

master data set contains a complete list of the inventory items. The transaction data set

contains items that are on the master inventory as well as new inventory items.

The following program creates the ADD_INVENTORY transaction data set, which

contains items for updating the master data set. The PartNumber variable contains the

part number for the item and corresponds to PartNumber in the INVENTORY_TOOL

data set. The Description variable names the item. The NewStock variable contains the

Modifying a Master Data Set with Observations from a Transaction Data Set 339