[INLONG-11233][SDK] Transform SQL supports mid function

inlong-sdk/transform-sdk/src/main/java/org/apache/inlong/sdk/transform/process/function/SubstringFunction.java

@@ -33,7 +33,7 @@
  * description:
  * return a substring of STRING starting from position INT1 with length INT2 (to the end by default)
  */
-@TransformFunction(names = {"substring", "substr"})
+@TransformFunction(names = {"substring", "substr", "mid"})
 public class SubstringFunction implements ValueParser {
 
     private ValueParser stringParser;

inlong-sdk/transform-sdk/src/test/java/org/apache/inlong/sdk/transform/process/function/string/TestSubstringFunction.java

@@ -93,5 +93,25 @@ public void testSubstringFunction() throws Exception {
         output = processor.transform(data, new HashMap<>());
         Assert.assertEquals(1, output.size());
         Assert.assertEquals("result=", output.get(0));
+
+        String transformSql3 = "select mid(string2, numeric1) from source";
+        TransformConfig config3 = new TransformConfig(transformSql3);
+        TransformProcessor<String, String> processor3 = TransformProcessor
+                .create(config3, SourceDecoderFactory.createCsvDecoder(csvSource),
+                        SinkEncoderFactory.createKvEncoder(kvSink));
+        // case7: mid('banana', 2)
+        List<String> output4 = processor3.transform("apple|banana|cloud|2|1|3", new HashMap<>());
+        Assert.assertEquals(1, output4.size());
+        Assert.assertEquals(output4.get(0), "result=anana");
+
+        String transformSql4 = "select mid(string1, numeric1, numeric3) from source";
+        TransformConfig config4 = new TransformConfig(transformSql4);
+        TransformProcessor<String, String> processor4 = TransformProcessor
+                .create(config4, SourceDecoderFactory.createCsvDecoder(csvSource),
+                        SinkEncoderFactory.createKvEncoder(kvSink));
+        // case8: mid('apple', 2, 3)
+        List<String> output5 = processor4.transform("apple|banana|cloud|2|1|3", new HashMap<>());
+        Assert.assertEquals(1, output5.size());
+        Assert.assertEquals(output5.get(0), "result=ppl");
     }
 }