Ensure sql_runif() actually returns random numbers

Manual merge of PR by by @thomashulst. Fixes #1503. Closes #1504

Author: hadley

NEWS.md

@@ -1,5 +1,7 @@
 # dbplyr (development version)
 
+* SQL server: `slice_sample()` returns different results each run (@thomashulst, #1503)
+* Corrected translation of `stringr::str_like()` to use case-sensitive `LIKE` when argument `ignore_case` is set as `FALSE` (@edward-burn, #1488).
 * Corrected translation of `stringr::str_like()` to use case-sensitive `LIKE` when argument `ignore_case` is set as `FALSE` (@edward-burn, #1488).
 * Fixed overwrite flag in `copy_to()` to work when source is in the same DB as destination (@liudvikasakelis, #1535)
 * Snowflake correctly translates `$` to `:` (@jsowder, #1608)

R/backend-mssql.R

@@ -322,7 +322,8 @@ simulate_mssql <- function(version = "15.0") {
       is.na = mssql_is_null,
 
       runif = function(n = n(), min = 0, max = 1) {
-        sql_runif(RAND(), n = {{ n }}, min = min, max = max)
+        # https://stackoverflow.com/a/9039661
+        sql_runif(RAND(CHECKSUM(NEWID())), n = {{ n }}, min = min, max = max)
       },
 
       # string functions ------------------------------------------------
@@ -747,5 +748,7 @@ utils::globalVariables(c(
   "BIGINT",
   "INT",
   "%AND%",
-  "%BETWEEN%"
+  "%BETWEEN%",
+  "CHECKSUM",
+  "NEWID"
 ))

tests/testthat/_snaps/backend-mssql.md

@@ -275,7 +275,7 @@
       <SQL>
       SELECT `x`
       FROM (
-        SELECT `df`.*, ROW_NUMBER() OVER (ORDER BY RAND()) AS `col01`
+        SELECT `df`.*, ROW_NUMBER() OVER (ORDER BY RAND(CHECKSUM(NEWID()))) AS `col01`
         FROM `df`
       ) AS `q01`
       WHERE (`col01` <= 1)