diff --git a/tests/system-test/2-query/diff.py b/tests/system-test/2-query/diff.py
index 30b588fa97025f4ee50f7f0e8bf2f07331e4711c..c6800d9a8a8ec4137551adcd2f79a4d1c8e58496 100644
--- a/tests/system-test/2-query/diff.py
+++ b/tests/system-test/2-query/diff.py
@@ -95,7 +95,6 @@ class TDTestCase:
         tdSql.error("select diff(col12) from stb_1")
         tdSql.error("select diff(col13) from stb_1")
         tdSql.error("select diff(col14) from stb_1")
-        tdSql.error("select ts,diff(col1),ts from stb_1")
 
         tdSql.query("select diff(col1) from stb_1")
         tdSql.checkRows(10)
@@ -115,6 +114,79 @@ class TDTestCase:
         tdSql.query("select diff(col6) from stb_1")
         tdSql.checkRows(10)
 
+        # check selectivity
+        tdSql.query("select ts, diff(col1), col2 from stb_1")
+        tdSql.checkRows(10)
+        tdSql.checkData(0, 0, "2018-09-17 09:00:00.000")
+        tdSql.checkData(1, 0, "2018-09-17 09:00:00.001")
+        tdSql.checkData(2, 0, "2018-09-17 09:00:00.002")
+        tdSql.checkData(3, 0, "2018-09-17 09:00:00.003")
+        tdSql.checkData(4, 0, "2018-09-17 09:00:00.004")
+        tdSql.checkData(5, 0, "2018-09-17 09:00:00.005")
+        tdSql.checkData(6, 0, "2018-09-17 09:00:00.006")
+        tdSql.checkData(7, 0, "2018-09-17 09:00:00.007")
+        tdSql.checkData(8, 0, "2018-09-17 09:00:00.008")
+        tdSql.checkData(9, 0, "2018-09-17 09:00:00.009")
+
+        tdSql.checkData(0, 1, 1)
+        tdSql.checkData(1, 1, 1)
+        tdSql.checkData(2, 1, 1)
+        tdSql.checkData(3, 1, 1)
+        tdSql.checkData(4, 1, 1)
+        tdSql.checkData(5, 1, 1)
+        tdSql.checkData(6, 1, 1)
+        tdSql.checkData(7, 1, 1)
+        tdSql.checkData(8, 1, 1)
+        tdSql.checkData(9, 1, 1)
+
+        tdSql.checkData(0, 2, 0)
+        tdSql.checkData(1, 2, 1)
+        tdSql.checkData(2, 2, 2)
+        tdSql.checkData(3, 2, 3)
+        tdSql.checkData(4, 2, 4)
+        tdSql.checkData(5, 2, 5)
+        tdSql.checkData(6, 2, 6)
+        tdSql.checkData(7, 2, 7)
+        tdSql.checkData(8, 2, 8)
+        tdSql.checkData(9, 2, 9)
+
+        tdSql.query("select ts, diff(col1), col2 from stb order by ts")
+        tdSql.checkRows(10)
+
+        tdSql.checkData(0, 0, "2018-09-17 09:00:00.000")
+        tdSql.checkData(1, 0, "2018-09-17 09:00:00.001")
+        tdSql.checkData(2, 0, "2018-09-17 09:00:00.002")
+        tdSql.checkData(3, 0, "2018-09-17 09:00:00.003")
+        tdSql.checkData(4, 0, "2018-09-17 09:00:00.004")
+        tdSql.checkData(5, 0, "2018-09-17 09:00:00.005")
+        tdSql.checkData(6, 0, "2018-09-17 09:00:00.006")
+        tdSql.checkData(7, 0, "2018-09-17 09:00:00.007")
+        tdSql.checkData(8, 0, "2018-09-17 09:00:00.008")
+        tdSql.checkData(9, 0, "2018-09-17 09:00:00.009")
+
+        tdSql.checkData(0, 1, 1)
+        tdSql.checkData(1, 1, 1)
+        tdSql.checkData(2, 1, 1)
+        tdSql.checkData(3, 1, 1)
+        tdSql.checkData(4, 1, 1)
+        tdSql.checkData(5, 1, 1)
+        tdSql.checkData(6, 1, 1)
+        tdSql.checkData(7, 1, 1)
+        tdSql.checkData(8, 1, 1)
+        tdSql.checkData(9, 1, 1)
+
+        tdSql.checkData(0, 2, 0)
+        tdSql.checkData(1, 2, 1)
+        tdSql.checkData(2, 2, 2)
+        tdSql.checkData(3, 2, 3)
+        tdSql.checkData(4, 2, 4)
+        tdSql.checkData(5, 2, 5)
+        tdSql.checkData(6, 2, 6)
+        tdSql.checkData(7, 2, 7)
+        tdSql.checkData(8, 2, 8)
+        tdSql.checkData(9, 2, 9)
+
+
         tdSql.execute('''create table stb1(ts timestamp, col1 tinyint, col2 smallint, col3 int, col4 bigint, col5 float, col6 double,
                     col7 bool, col8 binary(20), col9 nchar(20), col11 tinyint unsigned, col12 smallint unsigned, col13 int unsigned, col14 bigint unsigned) tags(loc nchar(20))''')
         tdSql.execute("create table stb1_1 using stb tags('shanghai')")
diff --git a/tests/system-test/2-query/function_diff.py b/tests/system-test/2-query/function_diff.py
index 5e95510c1d7337cfe81850249a375d35ba307c7c..99e87e6cd647a9731052014cc848d890d81c9362 100644
--- a/tests/system-test/2-query/function_diff.py
+++ b/tests/system-test/2-query/function_diff.py
@@ -283,14 +283,14 @@ class TDTestCase:
         tdSql.error(self.diff_query_form(alias=", diff(c1)"))   # mix with calculation function  2
         # tdSql.error(self.diff_query_form(alias=" + 2"))         # mix with arithmetic 1
         tdSql.error(self.diff_query_form(alias=" + avg(c1)"))   # mix with arithmetic 2
-        tdSql.error(self.diff_query_form(alias=", c2"))         # mix with other 1
+        tdSql.query(self.diff_query_form(alias=", c2"))         # mix with other 1
         # tdSql.error(self.diff_query_form(table_expr="stb1"))    # select stb directly
         stb_join = {
             "col": "stb1.c1",
             "table_expr": "stb1, stb2",
             "condition": "where stb1.ts=stb2.ts and stb1.st1=stb2.st2 order by stb1.ts"
         }
-        tdSql.error(self.diff_query_form(**stb_join))           # stb join
+        tdSql.query(self.diff_query_form(**stb_join))           # stb join
         interval_sql = {
             "condition": "where ts>0 and ts < now interval(1h) fill(next)"
         }