Combining data horizontally refers to the process of merging or joining multiple data sets into one data set. This process is referred to as a horizontal combination because, in the final data set, each observation (or horizontal row) will have variables from more than one input data set.

It is useful to combine data horizontally if you have several data sets that contain different but related information. For example, suppose you have one data set that contains employee data with the variables IDNumber, Name, and Address, and another data set that contains employee data with the variables IDNumber and Salary. You can combine the data from these two input data sets horizontally to create an output data set that contains IDNumber, Name, Address, and Salary.

This chapter focuses on several methods of combining data horizontally in the DATA step, and compares a DATA step match-merge with a PROC SQL join. This chapter also covers several techniques for horizontally combining data from an input data set with values that are not stored in a SAS data set.