diff --git a/teradata/api.py b/teradata/api.py
index 37ba01f..62d4be1 100644
--- a/teradata/api.py
+++ b/teradata/api.py
@@ -168,6 +168,21 @@ def __init__(self, code, msg):
 DateFromTicks = datetime.date.fromtimestamp
 
 
+class Interval(str):
+    """String representation of the Interval dataType."""
+    pass
+
+
+class JSON(str):
+    """String representation of the JSON dataType."""
+    pass
+
+
+class Period(str):
+    """String representation of the Period dataType."""
+    pass
+
+
 def TimeFromTicks(x):
     return datetime.datetime.fromtimestamp(x).time()
 
diff --git a/teradata/datatypes.py b/teradata/datatypes.py
index 702178a..098f10d 100644
--- a/teradata/datatypes.py
+++ b/teradata/datatypes.py
@@ -47,15 +47,15 @@
 periodRegEx1 = re.compile("\('(.*)',\s*'(.*)'\)")
 periodRegEx2 = re.compile("ResultStruct:PERIOD\(.*\)\[(.*),\s*(.*)\]")
 
-NUMBER_TYPES = ("BYTEINT", "BIGINT", "DECIMAL", "DOUBLE", "DOUBLE PRECISION",
+NUMBER_TYPES = {"BYTEINT", "BIGINT", "DECIMAL", "DOUBLE", "DOUBLE PRECISION",
                 "INTEGER", "NUMBER", "SMALLINT", "FLOAT", "INT", "NUMERIC",
-                "REAL")
+                "REAL"}
 
-INT_TYPES = ("BYTEINT", "BIGINT", "INTEGER", "SMALLINT", "INT")
+INT_TYPES = {"BYTEINT", "BIGINT", "INTEGER", "SMALLINT", "INT"}
 
-FLOAT_TYPES = ("FLOAT", "DOUBLE", "DOUBLE PRECISION", "REAL")
+FLOAT_TYPES = {"FLOAT", "DOUBLE", "DOUBLE PRECISION", "REAL"}
 
-BINARY_TYPES = ("BLOB", "BYTE", "VARBYTE")
+BINARY_TYPES = {"BLOB", "BYTE", "VARBYTE"}
 
 
 def _getMs(m, num):
@@ -230,7 +230,9 @@ def convertValue(self, dbType, dataType, typeCode, value):
         logger.trace(
             "Converting \"%s\" to (%s, %s).", value, dataType, typeCode)
         if value is not None:
-            if typeCode == NUMBER:
+            if typeCode == STRING:
+                return value
+            elif typeCode == NUMBER:
                 try:
                     return NUMBER(value)
                 except:
@@ -270,12 +272,12 @@ def convertValue(self, dbType, dataType, typeCode, value):
             elif typeCode == BINARY:
                 if util.isString(value):
                     return bytearray.fromhex(value)
-            elif dataType.startswith("INTERVAL"):
+            elif typeCode == Interval:
                 return convertInterval(dataType, value)
-            elif dataType.startswith("JSON") and util.isString(value):
+            elif typeCode == JSON and util.isString(value):
                 return json.loads(value, parse_int=decimal.Decimal,
                                   parse_float=decimal.Decimal)
-            elif dataType.startswith("PERIOD"):
+            elif typeCode == Period:
                 return convertPeriod(dataType, value)
         return value
 
@@ -294,6 +296,12 @@ def convertType(self, dbType, dataType):
             typeCode = Timestamp
         elif dataType.startswith("TIME"):
             typeCode = Time
+        elif dataType.startswith("INTERVAL"):
+            typeCode = Interval
+        elif dataType.startswith("JSON"):
+            typeCode = JSON
+        elif dataType.startswith("PERIOD"):
+            typeCode = Period
         return typeCode