huaxingao commented on a change in pull request #28264:
URL: https://github.com/apache/spark/pull/28264#discussion_r414689318
##########
File path: docs/sql-ref-datatypes.md
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@@ -706,3 +708,67 @@ The following table shows the type names as well as
aliases used in Spark SQL pa
</table>
</div>
</div>
+
+### Floating Point Special Values
+
+Spark SQL supports several special floating point values in a case-insensitive
manner:
+
+ * Inf/+Inf/Infinity/+Infinity: positive infinity
+ * ```FloatType```: equivalent to Scala <code>Float.PositiveInfinity</code>.
+ * ```DoubleType```: equivalent to Scala
<code>Double.PositiveInfinity</code>.
+ * -Inf/-Infinity: negative infinity
+ * ```FloatType```: equivalent to Scala <code>Float.NegativeInfinity</code>.
+ * ```DoubleType```: equivalent to Scala
<code>Double.NegativeInfinity</code>.
+ * NaN: not a number
+ * ```FloatType```: equivalent to Scala <code>Float.NaN</code>.
+ * ```DoubleType```: equivalent to Scala <code>Double.NaN</code>.
+
+#### Examples
+
+{% highlight sql %}
+SELECT double('infinity') AS col;
++--------+
+| col|
++--------+
+|Infinity|
++--------+
+
+SELECT float('-inf') AS col;
++---------+
+| col|
++---------+
+|-Infinity|
++---------+
+
+SELECT float('NaN') AS col;
++---+
+|col|
++---+
+|NaN|
++---+
+{% endhighlight %}
+
+### Positive/negative Infinity Semantics
+There is special handling for positive and negative infinity. They have the
following semantics:
+
+ - Positive infinity multiplied by any positive value returns positive
infinity.
+ - Negative infinity multiplied by any positive value returns negative
infinity.
+ - Positive infinity multiplied by any negative value returns negative
infinity.
+ - Negative infinity multiplied by any negative value returns positive
infinity.
+ - Positive infinity/Negtive infinity multiplied by 0 returns NaN.
+ - Infinity values equal to each other, no matter positive or negative.
Review comment:
I kind of feel we are saying "Positive infinity equals to negative
infinity" :)
I will change it to
```Positive/negative infinities are equal to themselves``` so it won't have
any ambiguity.
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