diff --git a/expression/builtin_compare.go b/expression/builtin_compare.go
index e2696356e2..5053454731 100644
--- a/expression/builtin_compare.go
+++ b/expression/builtin_compare.go
@@ -1092,6 +1092,8 @@ func getBaseCmpType(lhs, rhs types.EvalType, lft, rft *types.FieldType) types.Ev
 		return types.ETString
 	} else if (lhs == types.ETInt || (lft != nil && lft.Hybrid())) && (rhs == types.ETInt || (rft != nil && rft.Hybrid())) {
 		return types.ETInt
+	} else if (lhs == types.ETDecimal && rhs == types.ETString) || (lhs == types.ETString && rhs == types.ETDecimal) {
+		return types.ETReal
 	} else if ((lhs == types.ETInt || (lft != nil && lft.Hybrid())) || lhs == types.ETDecimal) &&
 		((rhs == types.ETInt || (rft != nil && rft.Hybrid())) || rhs == types.ETDecimal) {
 		return types.ETDecimal
diff --git a/expression/integration_test.go b/expression/integration_test.go
index b3242e1027..c61ac3f38f 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -8392,6 +8392,19 @@ func (s *testIntegrationSuite) TestIssue12205(c *C) {
 		testkit.Rows("Warning 1292 Truncated incorrect time value: '18446744072635875000'"))
 }
 
+// for issue 20128
+func (s *testIntegrationSerialSuite) TestIssue20128(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t;")
+	tk.MustExec("create table t(b enum('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z') DEFAULT NULL, c decimal(40,20));")
+	tk.MustExec("insert into t values('z', 19.18040000000000000000);")
+	tk.MustExec("insert into t values('z', 26.18040000000000000000);")
+	tk.MustExec("insert into t values('z', 25.18040000000000000000);")
+	tk.MustQuery("select * from t where t.b > t.c;").Check(testkit.Rows("z 19.18040000000000000000", "z 25.18040000000000000000"))
+	tk.MustQuery("select * from t where t.b < t.c;").Check(testkit.Rows("z 26.18040000000000000000"))
+}
+
 func (s *testIntegrationSuite) TestIssue21677(c *C) {
 	tk := testkit.NewTestKit(c, s.store)