Add string function split_part (#1451)

docs/documentation/cn/sql-reference/sql-functions/string-functions/split_part.md

@@ -0,0 +1,42 @@
+# split_part
+
+## Syntax
+
+`VARCHAR split_part(VARCHAR content, VARCHAR delimiter, INT field)`
+
+## Description
+
+根据分割符拆分字符串, 返回指定的分割部分(从一开始计数)。
+
+## Examples
+
+```
+mysql> select split_part("hello word", " ", 1);
++----------------------------------+
+| split_part('hello word', ' ', 1) |
++----------------------------------+
+| hello                            |
++----------------------------------+
+
+
+mysql> select split_part("hello word", " ", 2);
++----------------------------------+
+| split_part('hello word', ' ', 2) |
++----------------------------------+
+| word                             |
++----------------------------------+
+
+mysql> select split_part("2019年7月8号", "月", 1);
++-----------------------------------------+
+| split_part('2019年7月8号', '月', 1)     |
++-----------------------------------------+
+| 2019年7                                 |
++-----------------------------------------+
+
+mysql> select split_part("abca", "a", 1);
++----------------------------+
+| split_part('abca', 'a', 1) |
++----------------------------+
+|                            |
++----------------------------+
+```