Tuesday, February 3, 2015

Solve the Top N questions in SAS/SQL

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;

proc sort;
    by descending weight;
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
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
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;
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)
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

/* 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
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
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

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