[GitHub] [calcite] rubenada commented on a diff in pull request #3258: [CALCITE-5771] Apply two different NULL semantics for CONCAT function(enabled in MySQL、Postgresql、BigQuery and MSSQL)

["rubenada (via GitHub)" <gi...@apache.org>]

rubenada commented on code in PR #3258:
URL: https://github.com/apache/calcite/pull/3258#discussion_r1231883768


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core/src/main/java/org/apache/calcite/sql/fun/SqlLibraryOperators.java:
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@@ -831,16 +831,43 @@ static RelDataType deriveTypeSplit(SqlOperatorBinding operatorBinding,
       new SqlLikeOperator("NOT RLIKE", SqlKind.RLIKE, true, true);
 
   /** The "CONCAT(arg, ...)" function that concatenates strings.
-   * For example, "CONCAT('a', 'bc', 'd')" returns "abcd". */
-  @LibraryOperator(libraries = {MYSQL, POSTGRESQL, BIG_QUERY})
+   * For example, "CONCAT('a', 'bc', 'd')" returns "abcd".
+   *
+   * <p>It accepts at least 1 argument and returns null if any of
+   * the arguments is null. */
+  @LibraryOperator(libraries = {MYSQL, BIG_QUERY})
   public static final SqlFunction CONCAT_FUNCTION =
       SqlBasicFunction.create("CONCAT",
           ReturnTypes.MULTIVALENT_STRING_SUM_PRECISION_NULLABLE,
-          OperandTypes.repeat(SqlOperandCountRanges.from(2),
+          OperandTypes.repeat(SqlOperandCountRanges.from(1),
               OperandTypes.STRING),
           SqlFunctionCategory.STRING)
           .withOperandTypeInference(InferTypes.RETURN_TYPE);
 
+  /** The "CONCAT(arg, ...)" function that concatenates strings,
+   * which never returns null.
+   * For example, "CONCAT('a', 'bc', 'd')" returns "abcd".
+   *
+   * <p>If one of the arguments is null, it will be treated as empty string.
+   * "CONCAT('a', null)" returns "a".
+   * "CONCAT('a', null, 'b')" returns "ab".
+   *
+   * <p>Returns empty string only when all arguments are null.

Review Comment:
   Nit pick: this does not seem entirely correct, I assume it should be "Returns empty string only when all arguments are null *or the empty string"*, is that correct?
   ```
   "CONCAT('')" returns ""
   "CONCAT('', null, '')" returns ""
   ```
   Maybe the new tests added by this PR should have some scenarios with the empty string, just to cover these cases.



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