[GitHub] yanghua commented on a change in pull request #6432: [FLINK-9970] Add ASCII/CHR function for table/sql API

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yanghua commented on a change in pull request #6432: [FLINK-9970] Add ASCII/CHR function for table/sql API
URL: https://github.com/apache/flink/pull/6432#discussion_r205934831
 
 

 ##########
 File path: flink-libraries/flink-table/src/main/scala/org/apache/flink/table/runtime/functions/ScalarFunctions.scala
 ##########
 @@ -196,9 +196,35 @@ object ScalarFunctions {
     new String(data)
   }
 
+  /**
+    * Returns the numeric value of the leftmost character of the string str.
+    * Returns 0 if str is the empty string. Returns NULL if str is NULL.
+    */
+  def ascii(str: String): String = {
+    if (str == null) {
+      null
+    } else if (str.equals("")) {
+      ""
+    } else {
+      str.charAt(0).toByte.toString
+    }
+  }
+
   /**
     * Returns the base string decoded with base64.
     */
   def fromBase64(str: String): String = new String(Base64.decodeBase64(str))
 
+  /**
+    * Returns string contains a character which converts from a ASCII integer.
+    * If the ASCII less then 0 or greater than 255, return null.
+    */
+  def chr(ascii: Integer): String = {
 
 Review comment:
   hi @hequn8128 others has accepted and fixed except this. can you give a hint? you mean:
   
   * one function and the parameter support multiple types? if support I can you Either[Type1, Type2], but more than two, how to handle this? I am not very familiar with Scala.
   
   * multiple functions, one support a single type?
   
   * assume supported these types, the `OperandTypes.NUMERIC` can represent them?
   
   ```scala
   val CHR = new SqlFunction(
       "CHR",
       SqlKind.OTHER_FUNCTION,
       ReturnTypes.cascade(ReturnTypes.explicit(SqlTypeName.VARCHAR), SqlTypeTransforms.TO_NULLABLE),
       null,
       OperandTypes.NUMERIC,
       SqlFunctionCategory.STRING
     )
   ```

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