Add support for CROSS JOIN through full_join (#24)

CROSS JOIN will be generated whenever by = character()

Fixes tidyverse/dplyr#2924

Author: javierluraschi

NEWS.md

@@ -74,6 +74,9 @@
 
 ## New features
 
+* `full_join()` over non-overlapping columns `by = character()` translated to
+  `CROSS JOIN` (#2924).
+
 * `case_when()` now translates to SQL "CASE WHEN" (#2894)
 
 * `x %in% c(1)` now generates the same SQL as `x %in% 1` (#2898).

R/sql-generic.R

@@ -51,27 +51,19 @@ sql_join.DBIConnection <- function(con, x, y, vars, type = "inner", by = NULL, .
     inner = sql("INNER JOIN"),
     right = sql("RIGHT JOIN"),
     full = sql("FULL JOIN"),
+    cross = sql("CROSS JOIN"),
     stop("Unknown join type:", type, call. = FALSE)
   )
 
   select <- sql_join_vars(con, vars)
-
-  on <- sql_vector(
-    paste0(
-      sql_table_prefix(con, by$x, "TBL_LEFT"),
-      " = ",
-      sql_table_prefix(con, by$y, "TBL_RIGHT")
-    ),
-    collapse = " AND ",
-    parens = TRUE
-  )
+  on <- sql_join_tbls(con, by)
 
   # Wrap with SELECT since callers assume a valid query is returned
   build_sql(
     "SELECT ", select, "\n",
     "  FROM ", x, "\n",
     "  ", JOIN, " ", y, "\n",
-    "  ON ", on, "\n",
+    if (!is.null(on)) build_sql("  ON ", on, "\n") else NULL,
     con = con
   )
 }
@@ -108,6 +100,23 @@ sql_join_var <- function(con, alias, x, y) {
   }
 }
 
+sql_join_tbls <- function(con, by) {
+  on <- NULL
+  if (length(by$x) + length(by$y) > 0) {
+    on <- sql_vector(
+      paste0(
+        sql_table_prefix(con, by$x, "TBL_LEFT"),
+        " = ",
+        sql_table_prefix(con, by$y, "TBL_RIGHT")
+      ),
+      collapse = " AND ",
+      parens = TRUE
+    )
+  }
+
+  on
+}
+
 sql_coalesce <- function(...) {
   vars <- sql_vector(list(...), parens = FALSE, collapse = ", ")
   build_sql("coalesce(", vars, ")")

R/tbl-lazy.R

@@ -211,8 +211,16 @@ distinct_.tbl_lazy <- function(.data, ..., .dots = list(), .keep_all = FALSE) {
 add_op_join <- function(x, y, type, by = NULL, copy = FALSE,
                         suffix = c(".x", ".y"),
                         auto_index = FALSE, ...) {
+  by_intersect <- intersect(tbl_vars(x), tbl_vars(y))
+
+  if (length(by_intersect) == 0 && length(by) == 0 &&
+      identical(type, "full") && is.character(by)) {
+    type <- "cross"
+    by <- list(x = character(0), y = character(0))
+  } else {
+    by <- common_by(by, x, y)
+  }
 
-  by <- common_by(by, x, y)
   y <- auto_copy(
     x, y,
     copy = copy,

tests/testthat/test-joins.R

@@ -92,3 +92,8 @@ test_that("sql generated correctly for all sources", {
 
   expect_equal_tbls(xy)
 })
+
+test_that("full join is promoted to cross join for no overlapping variables", {
+  result <- df1 %>% full_join(df2, by = character()) %>% collect()
+  expect_equal(nrow(result), 25)
+})