Generate NULL when collapsing empty vectors in SQL

Fixes #318. Reverts #292.

DESCRIPTION

@@ -42,4 +42,4 @@ Config/Needs/website: bench, forcats, ggbeeswarm, ggplot2, R.utils,
 Config/testthat/edition: 3
 Encoding: UTF-8
 Roxygen: list(markdown = TRUE)
-RoxygenNote: 7.2.3.9000
+RoxygenNote: 7.3.1

NEWS.md

@@ -1,5 +1,8 @@
 # glue (development version)
 
+* `glue_sql("{var*}")` once again generates `NULL` if var is empty.  
+  This reverts #292. (#318).
+
 * The `.envir` argument to `glue()` and `glue_data()` really must be an
   environment now, as documented. Previously a list-ish object worked in some
   cases (by accident, not really by design). When you need to lookup values in

R/sql.R

@@ -20,8 +20,9 @@
 #' values in a query, however not every database driver supports them.
 #'
 #' If you place a `*` at the end of a glue expression the values will be
-#' collapsed with commas. This is useful for the [SQL IN Operator](https://www.w3schools.com/sql/sql_in.asp)
-#' for instance.
+#' collapsed with commas, or if there are no values, produce `NULL`.
+#' This is useful for (e.g.) the
+#' [SQL IN Operator](https://www.w3schools.com/sql/sql_in.asp).
 #' @inheritParams glue
 #' @seealso [glue_sql_collapse()] to collapse [DBI::SQL()] objects.
 #' @param .con \[`DBIConnection`]: A DBI connection object obtained from
@@ -223,13 +224,10 @@ sql_quote_transformer <- function(connection, .na) {
       }
     } else {
       res <- identity_transformer(text, envir)
-      if (length(res) == 0L) {
-        if (should_collapse) {
-          return("")
-        } else {
-          return(NULL)
-        }
+      if (length(res) == 0L && should_collapse) {
+        return(DBI::SQL("NULL"))
       }
+
       if (inherits(res, "SQL")) {
         if (should_collapse) {
           res <- glue_collapse(res, ", ")

tests/testthat/test-sql.R

@@ -49,12 +49,12 @@ describe("glue_sql", {
     expect_identical(glue_sql("{var*}", .con = con, var = "a"), DBI::SQL("'a'"))
     expect_identical(glue_sql("{var*}", .con = con, var = letters[1:5]), DBI::SQL("'a', 'b', 'c', 'd', 'e'"))
   })
-  it('collapses empty values to empty string', {
-    expect_identical(glue_sql("{var*}", .con = con, var = character()), DBI::SQL(""))
-    expect_identical(glue_sql("{var*}", .con = con, var = DBI::SQL(character())), DBI::SQL(""))
+  it('collapses empty values to NULLempty ', {
+    expect_identical(glue_sql("{var*}", .con = con, var = character()), DBI::SQL("NULL"))
+    expect_identical(glue_sql("{var*}", .con = con, var = DBI::SQL(character())), DBI::SQL("NULL"))
   })
   it("mimics glue() when not collapsing", {
-    expect_equal(glue_sql("{var}", var = NULL), DBI::SQL(glue("{var}", var = NULL)))
+    expect_equal(glue_sql("{var}", .con = con, var = NULL), DBI::SQL(glue("{var}", var = NULL)))
   })
 
   it("should return an SQL NULL by default for missing values", {