how to store SQL query result in data frame in shiny - r

Question

I am new to R and i am using shiny package to build a shiny app that can take different type of queries and those queries takes dynamic user id provided by users on ui level and also i want to show the demo of result of query on ui.
So, my problem is that i am not able to store the query results into an data frame also on clicking on Download button csv is not storing in my system. my code is below. thanks.
###server
library(shiny)
library(RMySQL)
shinyServer(function(input, output) {
datasetInput <- reactive({
switch(input$queryset,
"CDR" = cdr,
"ASSET" = ast,
"USAGE" = usg)
})
output$tbl <- renderTable({
conn <- dbConnect(drv = RMySQL::MySQL(),dbname = "xxxx",
host = "xxxxxx",
username = "xxxxx",
password = "xxxxx"),
q<-dbSendQuery(conn,paste0("select * from table where user_id='",input$user_id,"' and start_time >= '2016-07-16' and start_time < '2016-07-28' order by start_time limit 10 ;",sep = ""
))
dat<- dbFetch(q,n=-1)
on.exit(dbDisconnect(conn), add = TRUE)
})
output$view <- renderTable({
head({dat}, n = input$nrows)
})
output$downloadData <- downloadHandler(
filename = function() { paste(input$user_id, '.csv', sep='') },
content = function(file) {
write.csv({dat}, file)
})
}
)
###ui
library(shiny)
shinyUI(fluidPage(
titlePanel("My App"),
sidebarLayout(
sidebarPanel(
selectInput("queryset", "Choose the type of query:",
choices = c("CDR", "ASSET", "USAGE")),
numericInput("nrows", "Enter the no. of observations:", 10),
numericInput("user_id", "Enter user_id:", 0),
downloadButton('downloadData', 'Download',class = NULL)
),
mainPanel(
tableOutput("view")
)
)
))