Solve the Top N questions in SAS/SQL

This post was kindly contributed by DATA ANALYSIS - go there to comment and to read the full post.

This is a following post after my previous post about SAS/SQL.

SAS’s SQL procedure has a basic SQL syntax. I found that the most challenging work is to use PROC SQL to solve the TOP N (or TOP N by Group) questions. Comparing with other modern database systems, PROC SQL is lack of –

The ranking functions such as RANK() or the SELECT TOP clause such as
```
select TOP 3 * 
from class
;
```

The partition by clause such as

select sex, name, weight
from (select sex, name, max(weight) over(partition by sex) max_weight
    from class)
where  weight = max_weight
;

However, there are always some alternative solutions in SAS. I list a few question from an ascending difficulty below to explore the possibilities.

Prepare the data

First a SASHELP.CLASS dataset is used as a demo (availabe for every SAS copy). It is a small weight and height dataset from a faked class of 19 children. Now I only keep the weight variable as target column.

data class;
    set sashelp.class;
    keep name sex weight;
run;

proc sort;
    by descending weight;
run;

Name	Sex	Age	Weight
Philip	M	16	150
Ronald	M	15	133
Robert	M	12	128
Alfred	M	14	112.5
Janet	F	15	112.5
Mary	F	15	112
William	M	15	112
Carol	F	14	102.5
Henry	M	14	102.5
John	M	12	99.5
Barbara	F	13	98
Judy	F	14	90
Thomas	M	11	85
Jane	F	12	84.5
Alice	F	13	84
Jeffrey	M	13	84
James	M	12	83
Louise	F	12	77
Joyce	F	11	50.5

1. Select highest value

It is straightforward to use the outobs option at the begining to single out the highest weight.

title "Select highest weight overall";
proc sql outobs = 1;
    select name, weight
    from class
    order by weight desc
;quit;

Name	Weight
Philip	150

2. Select second highest value

How about the second highest weight? The logic is simple — if we remove the highest weight first, then the second highest weight will take the first row.

title "Select second highest weight overall";
proc sql outobs = 1;
    select name, weight 
    from class
    where weight not in (select max(weight) from class)
    order by weight desc
;quit;

Name	Weight
Ronald	133

3. Select Nth highest value

Now it comes to the hard part. How about the Nth highest value, say, the fourth highest weight? Now we have to do a self-joining to let the distinct value point to 3. Since there are two children with the weight 112.5, the query returns the two tied names.

title "Select Nth highest weight";
%let n = 4;

proc sql;
    select distinct a.name, a.weight
    from class as a
    where (select count(distinct b.weight)
        from class as b
        where b.weight > a.weight
        ) = &n - 1;
quit;

Name	Weight
Alfred	112.5
Janet	112.5

4. Select highest values by group

There are two groups Male and Female in the class, and the easiest way to find the highest weight for each category is select max for female union select max for male. However, a more scalable solution is to use the group by clause that fits more than two groups.

title "Select highest weights by group";
proc sql;
    select sex, name, weight
    from class
    group by sex
    having weight = max(weight)
;quit;

Sex	Name	Weight
F	Janet	112.5
M	Philip	150

5. Rank all values

The ultimate solution to solve all the question above is to derive a rank column for the target. There are two solutions: the first one use a subquery in the select clause, while the second one utilizes a subquery in the where clause.

The subquery in the first solution is independent to the main query, which uses less codes and is easier to recall in practice. The second one is actually a self-joining that is faster than the first solution.

/* Solution I */
proc sql; 
    select name, weight, (select count(distinct b.weight) 
            from class as b where b.weight >= a.weight) as Rank
    from class as a
    order by rank
;quit;

/* Solution II */
proc sql;
    select a.name, a.weight, count(b.weight) as rank
    from class as a, (select distinct weight
           from class
           ) as b
    where a.weight <= b.weight
    group by a.name, a.weight
    order by a.weight desc
;quit;

Name	Weight	Rank
Philip	150	1
Ronald	133	2
Robert	128	3
Alfred	112.5	4
Janet	112.5	4
Mary	112	5
William	112	5
Henry	102.5	6
Carol	102.5	6
John	99.5	7
Barbara	98	8
Judy	90	9
Thomas	85	10
Jane	84.5	11
Alice	84	12
Jeffrey	84	12
James	83	13
Louise	77	14
Joyce	50.5	15

6. Select top N values by group

Once with the rank column at hand, many perplexing problems could be easily solved. For example, we can use it to find the top 3 heaviest people for each category of male and female. And it is also scalable to more than two groups.

title "Select Top N weights by group";
proc sql; 
    select a.sex, a.name, a.weight, (select count(distinct b.weight) 
            from class as b where b.weight >= a.weight and a.sex = b.sex ) as rank 
    from class as a
    where calculated rank <= 3
    order by sex, rank
;quit;

Sex	Name	Weight	rank
F	Janet	112.5	1
F	Mary	112	2
F	Carol	102.5	3
M	Philip	150	1
M	Ronald	133	2
M	Robert	128	3