[R] class changed after execution with sqldf

[GL]

When I run sqldf to merge two datasets, it's changing the Date (class date)
to a numeric value (class factor). Not sure why. Appreciate any insight.
Console output for two datasets and the merged dataset (via sqldf) listed
below.


> summary(df.aggregate)
      Date                 Hour            x        
 Min.   :2010-07-01   0      :  64   Min.   : 0.00  
 1st Qu.:2010-07-25   1      :  64   1st Qu.: 1.00  
 Median :2010-08-16   2      :  64   Median : 9.00  
 Mean   :2010-08-16   3      :  64   Mean   :11.77  
 3rd Qu.:2010-09-08   4      :  64   3rd Qu.:23.00  
 Max.   :2010-09-30   5      :  64   Max.   :32.00  
                      (Other):1152                  
> class(df.aggregate$Date)
[1] "Date"
> summary(df.possible.combos)
      Date                 Hour      
 Min.   :2010-07-01   Min.   : 0.00  
 1st Qu.:2010-07-25   1st Qu.: 5.75  
 Median :2010-08-16   Median :11.50  
 Mean   :2010-08-16   Mean   :11.50  
 3rd Qu.:2010-09-08   3rd Qu.:17.25  
 Max.   :2010-09-30   Max.   :23.00  
> class(df.possible.combos$Date)
[1] "Date"
> #merge raw data and all possible combinations
>   df.final <- sqldf('select Date, Hour, x as RoomsInUse from
> "df.possible.combos"
+     left join "df.aggregate" using (Hour, Date)')
> summary(df.final)
      Date           Hour         RoomsInUse   
 14791.0:  24   Min.   : 0.00   Min.   : 0.00  
 14792.0:  24   1st Qu.: 5.75   1st Qu.: 1.00  
 14796.0:  24   Median :11.50   Median : 9.00  
 14797.0:  24   Mean   :11.50   Mean   :11.77  
 14798.0:  24   3rd Qu.:17.25   3rd Qu.:23.00  
 14799.0:  24   Max.   :23.00   Max.   :32.00  
 (Other):1392                                  
> class(df.final$Date)
[1] "factor"

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