Support TIME type and timediff function (#1505)

docs/documentation/cn/sql-reference/sql-functions/date-time-functions/datediff.md

@@ -2,7 +2,7 @@
 
 ## Syntax
 
-`DATETIME DATEDIFF(expr1,expr2)`
+`DATETIME DATEDIFF(DATETIME expr1,DATETIME expr2)`
 
 ## Description
 
@@ -28,4 +28,4 @@ mysql> select datediff(CAST('2010-11-30 23:59:59' AS DATETIME), CAST('2010-12-31
 +-----------------------------------------------------------------------------------+
 |                                                                               -31 |
 +-----------------------------------------------------------------------------------+
-```
+```

docs/documentation/cn/sql-reference/sql-functions/date-time-functions/timediff.md

@@ -0,0 +1,46 @@
+# timediff
+
+## Syntax
+
+`TIME TIMEDIFF(DATETIME expr1, DATETIME expr2)`
+
+## Description
+
+TIMEDIFF��������DATETIME֮���IJ�ֵ
+
+TIMEDIFF�������ر�ʾΪʱ��ֵ��expr1 - expr2�Ľ���������ֵΪTIME����
+
+�������������ڴ�-838:59:59��838:59:59֮����TIMEֵ��Χ��
+
+## Examples
+
+```
+mysql> SELECT TIMEDIFF(now(),utc_timestamp());
++----------------------------------+
+| timediff(now(), utc_timestamp()) |
++----------------------------------+
+| 08:00:00                         |
++----------------------------------+
+
+mysql> SELECT TIMEDIFF('2019-07-11 16:59:30','2019-07-11 16:59:21');
++--------------------------------------------------------+
+| timediff('2019-07-11 16:59:30', '2019-07-11 16:59:21') |
++--------------------------------------------------------+
+| 00:00:09                                               |
++--------------------------------------------------------+
+
+mysql> SELECT TIMEDIFF('2019-01-01 00:00:00', '2009-01-01 00:00:00');
++--------------------------------------------------------+
+| timediff('2019-01-01 00:00:00', '2009-01-01 00:00:00') |
++--------------------------------------------------------+
+| 838:59:59                                              |
++--------------------------------------------------------+
+
+mysql> SELECT TIMEDIFF('2019-01-01 00:00:00', NULL);
++---------------------------------------+
+| timediff('2019-01-01 00:00:00', NULL) |
++---------------------------------------+
+| NULL                                  |
++---------------------------------------+
+```
+