nested select in mysqlite

I am learning sqlite using the example from sqldf, I have come across example 3 about nested select and I don't quite understand the code:

sqldf("select iris.Species '[Species]',
              avg(Sepal_Length) '[Avg of SLs > avg SL]'
       from iris,
         (select Species, avg(Sepal_Length) SLavg
          from iris group by Species) SLavg
       where iris.Species = SLavg.Species
          and Sepal_Length > SLavg
       group by iris.Species")

I think the code above is trying to:

• create a table with [Species] and [Avg of SLs > avg SL] using data 开发者_运维问答from iris (from line 1-3)
• create a table named SLavg with Species and average Sepal_Length for each Species from iris (from line 4-6)

I wonder why SLavg is present in line 4-5 for 2 times, I tried removing SLavg and sqldf gives me statement: no such column: SLavg and statement: no such column: SLavg.Species.

The code queries two columns from cross-join of two tables:

select c1,c2 from iris,T2 where .... group by ....

Where T2 is a subquery which is given shorthand 'SLavg':

select d1, d2 from iris group by ...... // subquery

SLavg is used for both subquery name and column name, and that's confusing. Let's give them different names:

select 
    iris.Species '[Species]', avg(Sepal_Length) '[Avg of SLs > avg SL]'
from 
    iris, 
    (select Species, avg(Sepal_Length) AVGcol from iris group by Species) AVGtbl
where 
    iris.Species = AVGtbl.Species and Sepal_Length > AVGcol
group by 
    iris.Species

Now it should be more understandable:

• the subquery AVGtbl produces two columns (Species, AVGcol)
• the cross-join of iris and AVGtbl produces four columns (iris.Species, iris.Sepal_Length, AVGtbl.Species, AVGtbl.AVGcol).

The where clause of top-level select operates on these four.