Simple aggregation (summaries by year including other WHERE statements)
smry<- sqldf("SELECT fyear, avg(ni<0) AS pctloss FROM fnd WHERE ni IS NOT NULL AND fyear IS NOT NULL AND xrd IS NOT NULL and xrd>0 GROUP BY fyear")
library(ggplot2)
ggplot(smry, aes(fyear, pctloss)) +geom_point()+geom_line()
Merging (crsp.msi and crsp.msf)
sql <- paste("SELECT msi.date,msi.vwretd,msf.permno,msf.ret FROM crsp.msi LEFT JOIN crsp.msf ON msi.date>='2015-01-01' AND msi.date=msf.date")