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Showing content from https://winvector.github.io/rquery/reference/extend.html below:

Extend data by adding more columns. — extend • rquery

Create a node similar to a Codd extend relational operator (add derived columns).

extend(
  source,
  ...,
  partitionby = NULL,
  orderby = NULL,
  reverse = NULL,
  display_form = NULL,
  env = parent.frame()
)

extend_nse(
  source,
  ...,
  partitionby = NULL,
  orderby = NULL,
  reverse = NULL,
  display_form = NULL,
  env = parent.frame()
)

source to select from.

new column assignment expressions.

partitioning (window function) terms.

ordering (in window function) terms.

reverse ordering (in window function) terms.

chacter presentation form

environment to look for values in.

extend node.

Partitionby and orderby can only be used with a database that supports window-functions (such as PostgreSQL, Spark, and so on).

Supports bquote() .()-style name abstraction with the extenson that - promotes strings to names (please see here: https://github.com/WinVector/rquery/blob/master/Examples/Substitution/Substitution.md).

Note: if any window/aggregation functions are present then at least one of partitionby or orderby must be non empty. For this purpose partitionby=1 is allowed and means "single partition on the constant 1."

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))
  NEWCOL <- as.name("v")
  NEWVALUE = "zz"
  optree <- extend(d
, 
.(NEWCOL) %:=% ifelse(AUC>0.5
, 
R2
, 
1.0)
, 
.(NEWVALUE) %:=% 6)
  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")) %.>%
#>  extend(.,
#>   v := ifelse(AUC > 0.5, R2, 1),
#>   zz := 6)
#> SELECT
#>  `AUC`,
#>  `R2`,
#>  ( CASE WHEN ( `AUC` > 0.5 ) THEN ( `R2` ) WHEN NOT ( `AUC` > 0.5 ) THEN ( 1 ) ELSE NULL END )  AS `v`,
#>  6  AS `zz`
#> FROM (
#>  SELECT
#>   `AUC`,
#>   `R2`
#>  FROM
#>   `d`
#>  ) tsql_40096587785086276899_0000000000
#>   AUC  R2   v zz
#> 1 0.6 0.2 0.2  6

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