Re: [PR] [CALCITE-6431] Implement the SINGLE_VALUE aggregation in HiveSqlDiale… [calcite]

[via GitHub]

YiwenWu commented on code in PR #3817:
URL: https://github.com/apache/calcite/pull/3817#discussion_r1639114996


##########
core/src/main/java/org/apache/calcite/util/RelToSqlConverterUtil.java:
##########
@@ -93,6 +101,41 @@ public static void unparseSparkArrayAndMap(SqlWriter writer,
     writer.endList(frame);
   }
 
+
+  /**
+   * Rewrite SINGLE_VALUE use SparkSQL Mode.
+   *
+   * <blockquote><pre>
+   * CASE COUNT(*)
+   * WHEN 0 THEN NULL
+   * WHEN 1 THEN MIN(&lt;result&gt;)
+   * ELSE (SELECT NULL UNION ALL SELECT NULL))

Review Comment:
   1. A problem: `SELECT NULL UNION ALL SELECT NULL` will return two NULL 
values ​​instead of one.
   2. Suggestion: Use `COUNT(field)` instead of `COUNT(*)`



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