#' Query to drop columns from PostgreSQL database according df column names
#' @details Veergude kustutamine Postgresql andmebaasist etteantud df veergude nimede alusel.
#' Andmebaasist kustutatakse df veeru nimedega kokkulangevad andmebaasi veerud.
#' @param tbl The database table name.
#' @param df The DataFrame name.
#' @return query string.
#' @seealso \code{\link{queryDropColumnsFromPostgresDbAccordingDF}},
#' \code{\link{queryAddColumnsToPostgresDbAccordingDF}},
#' \code{\link{queryWithoutGeom}}
#' @examples \dontrun{
#' # Connect with database
#' tblName = "transport_tsoonid_tallinn"
#' q <- queryDropColumnsFromPostgresDbAccordingDF(df = tsoonAsumLosa, tbl = tblName)
#' con <- myPostgresConnect(host = conf$host, port = conf$port, user = conf$user,
#'         password = conf$password, dbname = conf$dbname, type = "RPostgreSQL")
#' res <- DBI::dbSendQuery(conn = con, statement = q)
#' DBI::dbClearResult(res)
#' DBI::dbDisconnect(con)
#'
#' }
#'
#' @export
#'

queryDropColumnsFromPostgresDbAccordingDF <- function(df, tbl) {
  colnames(df) <- dfNamesToLower(df = df)
  # Kustutame andmebaasist need veerud.
  res <- sapply(1:ncol(df), function(x) {
    paste0("DROP COLUMN IF EXISTS ", names(df)[x])
  })
  res <- paste(res, collapse = ", ")
  res <- paste0("ALTER TABLE ", tbl , " ", res, "")
  res
}