Hi SAS community,   I had a question about the data set step when I tried to re-assign new value to an existing variable. Below are 2 example codes. The second example codes work good and generate the data what we want. Somehow the first code cannot assign the value properly. And I would like to know the reason for why code 1 not working.   Example code 1:  ( I want to assign the all the non missing var2 from data d to missing value variable var1 from data b. However, in the test data, only the first non missing var2 from data d assigned to all the var1 missing value records.)   data test; set a b  c d; if b.var1='' and d.var2^='' then var1=var2; (variable 1 come from data b, while variable 2 come from data d) IF var1^=''; run;   Example code 2: (I separate the same code into 2 data step and the value can be assigned properly)   data test; set a b  c d; run;   data test; set test; if var1='' and var2^='' then var1=var2; (variable 1 come from data b, while variable 2 come from data d) IF var1^=''; run; ... View more