By default, DATA step match-merging combines all observations in all input data sets.

To exclude unmatched observations from your output data set, use the following in your DATA step:

To match-merge the data sets Cert.Demog and Cert.Visit and select only observations that appear in both data sets, use IN= to create two temporary variables, Indemog and Invisit. The IN= variable is a temporary variable that is available to program statements during the DATA step, but it is not included in the output SAS data set.

The DATA step that contains the IN= options appears below. The first IN= creates the temporary variable, Indemog, which is set to 1 when an observation from Cert.Demog contributes to the current observation. Otherwise, it is set to 0. Likewise, the value of Invisit depends on whether Cert.Visit contributes to an observation or not.

data work.merged; 
  merge cert.demog(in=indemog)  
    cert.visit(in=invisit
    rename=(date=BirthDate)); 
   by id; 
run;

To select only observations that appear in both Cert.Demog and Cert.Visit, specify a subsetting IF statement in the DATA step.

The subsetting IF statement checks the values of Indemog and Invisit and continues processing only those observations that meet the condition of the expression. The condition is that both Cert.Demog and Cert.Visit contribute to the observation. If the condition is met, the new observation is written to Work.Merged. Otherwise, the observation is deleted.

data work.merged; 
  merge cert.demog(in=indemog  
    rename=(date=BirthDate)) 
    cert.visit(in=invisit 
    rename=(date=VisitDate));  
   by id; 
   if indemog=1 and invisit=1; 
run; 
proc print data=work.merged; 
run;

In previous examples, Work.Merged contained 12 observations. In the output below, notice that only 10 observations met the condition in the IF expression.

SAS evaluates the expression within an IF statement to produce a result that is either nonzero, zero, or missing. A nonzero and nonmissing result causes the expression to be true; a zero or missing result causes the expression to be false.

It is possible to specify the subsetting IF statement from the previous example in either of the following ways. The first IF statement checks specifically for a value of 1. The second IF statement checks for a value that is neither missing nor 0 (which for IN= variables is always 1).

if indemog=1 and invisit=1;  

if indemog and invisit;