diff --git a/sqlor/filter.py b/sqlor/filter.py
index cecfb51..e41c7e6 100755
--- a/sqlor/filter.py
+++ b/sqlor/filter.py
@@ -34,6 +34,25 @@ except:
 	import json
 
 class DBFilter(object):
+	operators = [
+		"=",
+		"!=",
+		"IN",
+		"NOT IN",
+		">",
+		">=",
+		"<",
+		"<=",
+		"IS NULL",
+		"IS NOT NULL",
+		"LIKE",
+		"NOT LIKE"
+	]
+	logic = [
+		"AND",
+		"OR",
+		"NOT"
+	]
 	def __init__(self,filterjson):
 		if isinstance(filterjson, str):
 			filterjson = json.loads(filterjson)
@@ -103,9 +122,12 @@ class DBFilter(object):
 		keys = fj.keys()
 		assert 'field' in keys
 		assert 'op' in keys
-		assert 'const' in keys or 'var' in keys
-		op = fj.get('op')
-		assert op in ['=','<>','>','<','>=','<=','in','not in']
+		assert 'const' in keys or 'var' in keys  or fj['op'].upper() in ['IS NULL', 'IS NOT NULL']
+		op = fj.get('op').upper()
+		assert op in self.operators
+
+		if op in  ['IS NULL', 'IS NOT NULL']:
+			return f'{fj['field']} {op}'
 
 		var = fj.get('var')
 		if var and not var in ns.keys():
@@ -128,6 +150,11 @@ def default_filterjson(fields: list, ns: dict):
 		'op':'=',
 		'var':k
 		} for k in ns.keys() if k in fields ]
+	print(f'default_filterjson():{c=}')
+	if len(c) < 1:
+		return None
+	if len(c) < 2:
+		return c[0]
 	return {
 		'AND':c
 	}