Support append_trailing_char_if_absent function (#3439)

docs/en/sql-reference/sql-functions/string-functions/append_trailing_char_if_absent.md

@@ -0,0 +1,60 @@
+---
+{
+    "title": "append_trailing_char_if_absent",
+    "language": "en"
+}
+---
+
+<!-- 
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
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+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
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+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+# append_trailing_char_if_absent
+
+## description
+
+### Syntax
+
+`VARCHAR append_trailing_char_if_absent(VARCHAR str, VARCHAR trailing_char)`
+
+If the s string is non-empty and does not contain the c character at the end, it appends the c character to the end.
+Trailing_char contains only one character, and it will return NULL if contains more than one character
+
+## example
+
+```
+MySQL [test]> select append_trailing_char_if_absent('a','c');
++------------------------------------------+
+| append_trailing_char_if_absent('a', 'c') |
++------------------------------------------+
+| ac                                       |
++------------------------------------------+
+1 row in set (0.02 sec)
+
+MySQL [test]> select append_trailing_char_if_absent('ac','c');
++-------------------------------------------+
+| append_trailing_char_if_absent('ac', 'c') |
++-------------------------------------------+
+| ac                                        |
++-------------------------------------------+
+1 row in set (0.00 sec)
+```
+
+## keyword
+
+APPEND_TRAILING_CHAR_IF_ABSENT