Add new tamper script which can replaces instances like 'IFNULL(A, B)...
diff --git a/tamper/ifnull2casewhenisnull.py b/tamper/ifnull2casewhenisnull.py
new file mode 100644
index 00000000000..cabea4cc8f5
--- /dev/null
+++ b/tamper/ifnull2casewhenisnull.py
@@ -0,0 +1,65 @@
+#!/usr/bin/env python
+
+"""
+Copyright (c) 2006-2017 sqlmap developers (http://sqlmap.org/)
+See the file 'doc/COPYING' for copying permission
+"""
+
+from lib.core.enums import PRIORITY
+
+__priority__ = PRIORITY.HIGHEST
+
+def dependencies():
+    pass
+
+def tamper(payload, **kwargs):
+    """
+    Replaces instances like 'IFNULL(A, B)' with 'CASE WHEN ISNULL(A) THEN (B) ELSE (A) END'
+
+    Requirement:
+        * MySQL
+        * SQLite (possibly)
+        * SAP MaxDB (possibly)
+
+    Tested against:
+        * MySQL 5.0 and 5.5
+
+    Notes:
+        * Useful to bypass very weak and bespoke web application firewalls
+          that filter the IFNULL() and IF() functions 
+
+    >>> tamper('IFNULL(1, 2)')
+    'CASE WHEN ISNULL(1) THEN (2) ELSE (1) END'
+    """
+
+    if payload and payload.find("IFNULL") > -1:
+        while payload.find("IFNULL(") > -1:
+            index = payload.find("IFNULL(")
+            depth = 1
+            comma, end = None, None
+
+            for i in xrange(index + len("IFNULL("), len(payload)):
+                if depth == 1 and payload[i] == ',':
+                    comma = i
+
+                elif depth == 1 and payload[i] == ')':
+                    end = i
+                    break
+
+                elif payload[i] == '(':
+                    depth += 1
+
+                elif payload[i] == ')':
+                    depth -= 1
+
+            if comma and end:
+                _ = payload[index + len("IFNULL("):comma]
+                __ = payload[comma + 1:end].lstrip()
+                newVal = "CASE WHEN ISNULL(%s) THEN (%s) ELSE (%s) END" % (_, __, _)
+                payload = payload[:index] + newVal + payload[end + 1:]
+            else:
+                break
+
+    return payload
+
+

Hello, I created very similar tamper script like already exists here https://github.com/sqlmapproject/sqlmap/blob/master/tamper/ifnull2ifisnull.py

In my case WAF blocked my payloads if there are exists IFNULL() and IF() functions, so I just little modified **ifnull2ifisnull** tamper script.

> tamper('IFNULL(1, 2)')
'CASE WHEN ISNULL(1) THEN (2) ELSE (1) END'

I guess it could be useful for other too.