MaxGekk commented on a change in pull request #26034: [SPARK-29364][SQL] Return 
an interval from date subtract according to SQL standard
URL: https://github.com/apache/spark/pull/26034#discussion_r331764340
 
 

 ##########
 File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/datetimeExpressions.scala
 ##########
 @@ -1672,33 +1673,59 @@ case class TruncTimestamp(
 }
 
 /**
- * Returns the number of days from startDate to endDate.
+ * Returns the number of days from startDate to endDate or an interval between 
the dates.
  */
+// scalastyle:off line.size.limit line.contains.tab
 @ExpressionDescription(
-  usage = "_FUNC_(endDate, startDate) - Returns the number of days from 
`startDate` to `endDate`.",
+  usage = "_FUNC_(endDate, startDate) - Returns the number of days from 
`startDate` to `endDate`." +
+    "When `spark.sql.ansi.enabled` is set to `true` and `spark.sql.dialect` is 
`Spark`, it returns " +
+    "an interval between `startDate` (inclusive) and `endDate` (exclusive).",
   examples = """
     Examples:
       > SELECT _FUNC_('2009-07-31', '2009-07-30');
        1
-
       > SELECT _FUNC_('2009-07-30', '2009-07-31');
        -1
+      > SET spark.sql.ansi.enabled=true;
+      spark.sql.ansi.enabled   true
+      > SET spark.sql.dialect=Spark;
+      spark.sql.dialect        Spark
+      > select _FUNC_(date'tomorrow', date'yesterday');
+      interval 2 days
   """,
   since = "1.5.0")
+// scalastyle:on line.size.limit line.contains.tab
 case class DateDiff(endDate: Expression, startDate: Expression)
   extends BinaryExpression with ImplicitCastInputTypes {
 
   override def left: Expression = endDate
   override def right: Expression = startDate
   override def inputTypes: Seq[AbstractDataType] = Seq(DateType, DateType)
-  override def dataType: DataType = IntegerType
+  private val returnInterval: Boolean = {
+    val isSparkDialect = SQLConf.get.getConf(DIALECT) == 
Dialect.SPARK.toString()
+    SQLConf.get.ansiEnabled && isSparkDialect
 
 Review comment:
   > Please note that I suggested `Dialect.POSTGRESQL`
   
   If you do what you suggest, this will change results of `date.sql`:
   ```diff
   -- !query 46
    SELECT f1 - date '2000-01-01' AS `Days From 2K` FROM DATE_TBL
    -- !query 46 schema
   -struct<Days From 2K:int>
   +struct<Days From 2K:interval>
    -- !query 46 output
   --1035
   --1036
   --1037
   --1400
   --1401
   --1402
   --1403
   --15542
   --15607
   -13977
   -14343
   -14710
   -91
   -92
   -93
   +interval -2 years -10 months
   +interval -2 years -10 months -1 days
   +interval -2 years -9 months -4 weeks -2 days
   +interval -3 years -10 months
   +interval -3 years -10 months -1 days
   +interval -3 years -10 months -2 days
   +interval -3 years -9 months -4 weeks -2 days
   +interval -42 years -6 months -2 weeks -4 days
   +interval -42 years -8 months -3 weeks -1 days
   +interval 3 months
   +interval 3 months 1 days
   +interval 3 months 2 days
   +interval 38 years 3 months 1 weeks
   +interval 39 years 3 months 1 weeks 1 days
   +interval 40 years 3 months 1 weeks 2 days
   ```

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