Make an orderby node (not a relational operation).

Order a table by a set of columns (not general expressions) and limit number of rows in that order.

order_rows(
  source,
  cols = NULL,
  ...,
  reverse = NULL,
  limit = NULL,
  env = parent.frame()
)

Arguments

source	source to select from.
cols	order by columns ascending.
...	force later arguments to be bound by name
reverse	character, which columns to reverse ordering of to descending.
limit	number limit row count.
env	environment to look to.

Value

order_by node.

Details

Note: this is a relational operator in that it takes a table that is a relation (has unique rows) to a table that is still a relation. However, most relational systems do not preserve row order in storage or between operations. So without the limit set this is not a useful operator except as a last step prior to pulling data to an in-memory data.frame ( which does preserve row order).

Examples

if (requireNamespace("DBI", quietly = TRUE) && requireNamespace("RSQLite", quietly = TRUE)) {
  my_db <- DBI::dbConnect(RSQLite::SQLite(), ":memory:")
  d <- rq_copy_to(my_db, 'd',
                   data.frame(AUC = 0.6, R2 = 0.2))
  optree <- order_rows(d, cols = "AUC", reverse = "AUC", limit=4)
  cat(format(optree))
  sql <- to_sql(optree, my_db)
  cat(sql)
  print(DBI::dbGetQuery(my_db, sql))
  DBI::dbDisconnect(my_db)
}
#> mk_td("d", c(
#>   "AUC",
#>   "R2")) %.>%
#>  order_rows(.,
#>   c('AUC'),
#>   reverse = c('AUC'),
#>   limit = 4)
#> SELECT * FROM (
#>  SELECT
#>   `AUC`,
#>   `R2`
#>  FROM
#>   `d`
#> ) tsql_40259561509577993997_0000000000 ORDER BY `AUC` DESC LIMIT 4
#>   AUC  R2
#> 1 0.6 0.2