Add doctest sample.

Signed-off-by: Yury-Fridlyand <yury.fridlyand@improving.com>

docs/user/dql/expressions.rst

@@ -153,6 +153,30 @@ Here is an example for different type of comparison operators::
 It is possible to compare datetimes. When comparing different datetime types, for example `DATE` and `TIME`, both converted to `DATETIME`.
 The following rule is applied on coversion: a `TIME` applied to today's date; `DATE` is interpreted at midnight.
 
+    os> SELECT current_time() > current_date() AS `now.time > today`, typeof(current_time()) AS `now.time.type`, typeof(current_date()) AS `now.date.type`;
+    fetched rows / total rows = 1/1
+    +--------------------+-----------------+-----------------+
+    | now.time > today   | now.time.type   | now.date.type   |
+    +--------------------+-----------------+-----------------+
+    | True               | TIME            | DATE            |
+    +--------------------+-----------------+-----------------+
+
+    os> SELECT current_time() = now() AS `now.time = now`, typeof(current_time()) AS `now.time.type`, typeof(now()) AS `now.type`;
+    fetched rows / total rows = 1/1
+	+------------------+-----------------+------------+
+    | now.time = now   | now.time.type   | now.type   |
+    +------------------+-----------------+------------+
+    | True             | TIME            | DATETIME   |
+    +------------------+-----------------+------------+
+
+    os> SELECT subtime(now(), current_time()) = current_date() AS `midnight = now.date`, typeof(subtime(now(), current_time())) AS `midnight.type`, typeof(current_date()) AS `now.date.type`;
+    fetched rows / total rows = 1/1
+	+--------------------+-----------------+-----------------+
+    | midnight = today   | midnight.type   | now.date.type   |
+    +--------------------+-----------------+-----------------+
+    | True               | DATETIME        | DATE            |
+    +--------------------+-----------------+-----------------+
+
 
 LIKE
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