Translate pmin()/pmax() to LEAST()/GREATEST()

Except for SQLite (which uses MIN()/MAX()) and MS SQL (which errors)

Fixes #118.

NEWS.md

@@ -1,5 +1,9 @@
 # dbplyr (development version)
 
+* Default translation for `pmin()` and `pmax()` becomes `LEAST()` and 
+  `GREATEST()` (#118). SQLite instead uses `MIN()` and `MAX()`, and MS SQL 
+  throws an error.
+
 * Default translator gains translations for `paste()`, `paste0()`, 
   and the hyperbolic functions (these previously were only available for 
   ODBC based translations).

R/db-odbc-mssql.R

@@ -72,6 +72,11 @@
       atan2         = sql_prefix("ATN2"),
       ceil          = sql_prefix("CEILING"),
       ceiling       = sql_prefix("CEILING"),
+
+      # https://dba.stackexchange.com/questions/187090
+      pmin          = sql_not_supported("pmin()"),
+      pmax          = sql_not_supported("pmax()"),
+
       substr        = function(x, start, stop) {
                         len <- stop - start + 1
                         sql_expr(SUBSTRING(!!x, !!start, !!len))

R/db-sqlite.r

@@ -24,7 +24,11 @@ sql_translate_env.SQLiteConnection <- function(con) {
       },
       na_if = sql_prefix("NULLIF", 2),
       paste = sql_paste_infix(" ", "||", function(x) sql_expr(cast(!!x %as% text))),
-      paste0 = sql_paste_infix("", "||", function(x) sql_expr(cast(!!x %as% text)))
+      paste0 = sql_paste_infix("", "||", function(x) sql_expr(cast(!!x %as% text))),
+      # https://www.sqlite.org/lang_corefunc.html#maxoreunc
+      pmin = sql_prefix("MIN"),
+      pmax = sql_prefix("MAX"),
+
     ),
     sql_translator(.parent = base_agg,
       sd = sql_aggregate("stdev", "sd")

R/translate-sql-base.r

@@ -167,8 +167,8 @@ base_scalar <- sql_translator(
     sql_expr(!!x %BETWEEN% !!left %AND% !!right)
   },
 
-  pmin = sql_prefix("min"),
-  pmax = sql_prefix("max"),
+  pmin = sql_prefix("LEAST"),
+  pmax = sql_prefix("GREATEST"),
 
   `%>%` = `%>%`,
 

tests/testthat/test-translate-sqlite.R

@@ -10,6 +10,15 @@ test_that("vectorised translations", {
   expect_equal(trans(paste0(x, y)), sql("`x` || `y`"))
 })
 
+test_that("pmin and max become MIN and MAX", {
+  trans <- function(x) {
+    translate_sql(!!enquo(x), con = simulate_sqlite(), window = FALSE)
+  }
+
+  expect_equal(trans(pmin(x, y)), sql('MIN(`x`, `y`)'))
+  expect_equal(trans(pmax(x, y)), sql('MAX(`x`, `y`)'))
+})
+
 test_that("as.numeric()/as.double() get custom translation", {
   mf <- dbplyr::memdb_frame(x = 1L)
 

tests/testthat/test-translate.r

@@ -53,11 +53,6 @@ test_that("all forms of if translated to case statement", {
   expect_equal(translate_sql(if_else(x, 1L, 2L)), expected)
 })
 
-test_that("pmin and pmax become min and max", {
-  expect_equal(translate_sql(pmin(x, y)), sql('MIN("x", "y")'))
-  expect_equal(translate_sql(pmax(x, y)), sql('MAX("x", "y")'))
-})
-
 test_that("%in% translation parenthesises when needed", {
   expect_equal(translate_sql(x %in% 1L), sql('"x" IN (1)'))
   expect_equal(translate_sql(x %in% c(1L)), sql('"x" IN (1)'))
@@ -114,6 +109,12 @@ test_that("hypergeometric functions use manual calculation", {
   expect_equal(translate_sql(coth(x)), sql('(EXP(2 * ("x")) + 1) / (EXP(2 * ("x")) - 1)'))
 })
 
+
+test_that("pmin and max use GREATEST and LEAST", {
+  expect_equal(translate_sql(pmin(x, y)), sql('LEAST("x", "y")'))
+  expect_equal(translate_sql(pmax(x, y)), sql('GREATEST("x", "y")'))
+})
+
 test_that("round uses integer digits", {
   expect_equal(translate_sql(round(10.1)), sql("ROUND(10.1, 0)"))
   expect_equal(translate_sql(round(10.1, digits = 1)),  sql("ROUND(10.1, 1)"))