Restore previous NULL behaviour in operators (#1457)

Fixes #1436

NEWS.md

@@ -1,5 +1,8 @@
 # dbplyr (development version)
 
+* You can once again use `NULL` on the LHS of an infix operator in order
+  to generate SQL with unusual syntax (#1345).
+
 * Oracle once again translates `head()` to `FETCH FIRST`. This does require
   Oracle 12c or newer, but it actually works, compared to the approach using `ROWNUM`
   from #1292 (#1436).

R/translate-sql-helpers.R

@@ -153,29 +153,34 @@ sql_infix <- function(f, pad = TRUE) {
   # This is fixed with `escape_infix_expr()`
   # see https://github.com/tidyverse/dbplyr/issues/634
   check_string(f)
+  check_bool(pad)
   f <- sql(f)
 
-  if (pad) {
-    function(x, y) {
-      x <- escape_infix_expr(enexpr(x), x)
-      y <- escape_infix_expr(enexpr(y), y)
-
-      glue_sql2(sql_current_con(), "{.val x} {f} {.val y}")
-    }
-  } else {
-    function(x, y) {
-      x <- escape_infix_expr(enexpr(x), x)
-      y <- escape_infix_expr(enexpr(y), y)
-
-      glue_sql2(sql_current_con(), "{.val x}{f}{.val y}")
+  function(x, y) {
+    x <- escape_infix_expr(enexpr(x), x)
+    y <- escape_infix_expr(enexpr(y), y)
+
+    if (is.null(x)) {
+      if (pad) {
+        sql <- "{f} {.val y}"
+      } else {
+        sql <- "{f}{.val y}"
+      }
+    } else {
+      if (pad) {
+        sql <- "{.val x} {f} {.val y}"
+      } else {
+        sql <- "{.val x}{f}{.val y}"
+      }
     }
+    glue_sql2(sql_current_con(), sql)
   }
 }
 
 escape_infix_expr <- function(xq, x, escape_unary_minus = FALSE) {
   infix_calls <- c("+", "-", "*", "/", "%%", "^")
   is_infix <- is_call(xq, infix_calls, n = 2)
-  is_unary_minus <- escape_unary_minus && 
+  is_unary_minus <- escape_unary_minus &&
     is_call(xq, "-", n = 1) && !is_atomic(x, n = 1)
 
   if (is_infix || is_unary_minus) {

tests/testthat/test-translate-sql-helpers.R

@@ -66,11 +66,15 @@ test_that("unary minus works with expressions", {
   expect_equal(test_translate_sql(--x), sql("--`x`"))
 })
 
-test_that("pad = FALSE works", {
+test_that("sql_infix generates expected output (#1345)", {
   local_con(simulate_dbi())
-  subset <- sql_infix(".", pad = FALSE)
+  x <- ident_q("x")
+  y <- ident_q("y")
 
-  expect_equal(subset(ident("df"), ident("x")), sql("`df`.`x`"))
+  expect_equal(sql_infix("-", pad = FALSE)(x, y), sql("x-y"))
+  expect_equal(sql_infix("-", pad = FALSE)(NULL, y), sql("-y"))
+  expect_equal(sql_infix("-")(x, y), sql("x - y"))
+  expect_equal(sql_infix("-")(NULL, y), sql("- y"))
 })
 
 test_that("sql_prefix checks arguments", {