if last.byvar, 只保留byvar（你的code中为dept）的最后一个观测：即只保留根据dept sort后的最后一个观测。参考如下code

The subsetting IF statementcauses the DATA step to continue processing only those raw data recordsor those observations from a SAS data set that meet the conditionof the expression that is specified in the IF statement. That is,if the expression is true for the observation or record (its valueis neither 0 nor missing), SAS continues to execute statements inthe DATA step and includes the current observation in the data set.The resulting SAS data set or data sets contain a subset of the originalexternal file or SAS data set.

If the expression isfalse (its value is 0 or missing), no further statements are processedfor that observation or record, the current observation is not writtento the data set, and the remaining program statements in the DATAstep are not executed. SAS immediately returns to the beginning ofthe DATA step because the subsetting IF statement does not requireadditional statements to stop processing observations.