SAS_MergeVsModifyVsUpdate

Q)Descibe Update and Modify statements?
Q)Compare MERGE vs UPDATE vs MODIFY

Definition of Updating
Updating a SAS data set replaces the values of variables in master data set with values from transaction data set.
If the UPDATEMODE= option in UPDATE statement is set to MISSINGCHECK, then missing values in a transaction dataset don’t replace existing values in a master dataset.(default setting is MISSINGCHECK)

We update a data set by using the UPDATE statement along with a BY statement. Both of the input data sets must be sorted by the variable that we use in the BY statement.
DATA master;
Update master transaction;
By fieldname
RUN;

Definition of MODIFYING

Modifying a SAS data set replaces, deletes, or appends observations in an existing data set. Modifying a SAS data set is similar to updating a SAS data set, but some differences exist.
Modify statement Control the Update Process using REPLACE,REMOVE and OUTPUT statements.
When you use the MODIFY statement, there is an implied REPLACE statement at the bottom of the DATA step instead of an OUTPUT statement.

Using the MODIFY statement, we can update

every observation in a data set	DATA SAS-data-set; MODIFY SAS-data-set; existing-variable = expression; RUN;
observations using a transaction data set and a BY statement	DATA SAS-data-set; MODIFY SAS-data-set transaction-data-set; BY key-variable; RUN; MODIFY master-data-set transaction-data-set UPDATEMODE=MISSINGCHECK|NOMISSINGCHECK;
observations located using an index.	MODIFY SAS-data-set KEY=index-name;

Merge Vs Update Vs Modify

Criterion	MERGE	UPDATE	MODIFY
Can Create new dataset	When we submit a DATA step to create a SAS data set that is also named in a MERGE, UPDATE, or SET statement, SAS creates a second copy of the input data set.Once execution is done, SAS deletes the original copy of the data set. As a result,the original data set is replaced by the new data set.	When we submit a DATA step to create a SAS data set that is also named in a MERGE, UPDATE, or SET statement, SAS creates a second copy of the input data set.Once execution is done, SAS deletes the original copy of the data set. As a result,the original data set is replaced by the new data set.	when we submit a DATA step to create a SAS data set that is also named in the MODIFY statement, SAS does not create a second copy of data but instead updates the data set in place.
Can create or delete variables	Yes   The new data set can contain a different set of variables than the original data set and the attributes of the variables in the new data set can be different from those of the original data set.	Yes   The new data set can contain a different set of variables than the original data set and the attributes of the variables in the new data set can be different from those of the original data set.	No   Any variables can be added to PDV, but they are not written to the data set. So, the set of variables in the data set does not change when the data is modified.
Data sets must be sorted or indexed	Match-merge: Yes One-to-one merge: No	Yes	No
BY values must be unique	No	Master data set: Yes Transaction data set: No	No
Number of data sets combined	Any number	2	2
Processing missing values	Overwrites nonmissing values from first data set with missing values from second data set	Depends on the value of the UPDATEMODE= option	Depends on value of the UPDATEMODE= option