diff --git a/executor/aggfuncs/func_group_concat.go b/executor/aggfuncs/func_group_concat.go
index c2d3b745d12fc..7d892adb54a5c 100644
--- a/executor/aggfuncs/func_group_concat.go
+++ b/executor/aggfuncs/func_group_concat.go
@@ -26,8 +26,8 @@ import (
 	"github.com/pingcap/tidb/types"
 	"github.com/pingcap/tidb/util/chunk"
 	"github.com/pingcap/tidb/util/codec"
+	"github.com/pingcap/tidb/util/collate"
 	"github.com/pingcap/tidb/util/dbterror"
-	"github.com/pingcap/tidb/util/hack"
 	"github.com/pingcap/tidb/util/set"
 )
 
@@ -175,10 +175,16 @@ func (e *groupConcatDistinct) ResetPartialResult(pr PartialResult) {
 func (e *groupConcatDistinct) UpdatePartialResult(sctx sessionctx.Context, rowsInGroup []chunk.Row, pr PartialResult) (err error) {
 	p := (*partialResult4GroupConcatDistinct)(pr)
 	v, isNull := "", false
+
+	collators := make([]collate.Collator, 0, len(e.args))
+	for _, arg := range e.args {
+		collators = append(collators, collate.GetCollator(arg.GetType().Collate))
+	}
+
 	for _, row := range rowsInGroup {
 		p.valsBuf.Reset()
 		p.encodeBytesBuffer = p.encodeBytesBuffer[:0]
-		for _, arg := range e.args {
+		for i, arg := range e.args {
 			v, isNull, err = arg.EvalString(sctx, row)
 			if err != nil {
 				return err
@@ -186,7 +192,7 @@ func (e *groupConcatDistinct) UpdatePartialResult(sctx sessionctx.Context, rowsI
 			if isNull {
 				break
 			}
-			p.encodeBytesBuffer = codec.EncodeBytes(p.encodeBytesBuffer, hack.Slice(v))
+			p.encodeBytesBuffer = codec.EncodeBytes(p.encodeBytesBuffer, collators[i].Key(v))
 			p.valsBuf.WriteString(v)
 		}
 		if isNull {
@@ -478,10 +484,15 @@ func (e *groupConcatDistinctOrder) UpdatePartialResult(sctx sessionctx.Context,
 	p := (*partialResult4GroupConcatOrderDistinct)(pr)
 	p.topN.sctx = sctx
 	v, isNull := "", false
+	collators := make([]collate.Collator, 0, len(e.args))
+	for _, arg := range e.args {
+		collators = append(collators, collate.GetCollator(arg.GetType().Collate))
+	}
+
 	for _, row := range rowsInGroup {
 		buffer := new(bytes.Buffer)
 		p.encodeBytesBuffer = p.encodeBytesBuffer[:0]
-		for _, arg := range e.args {
+		for i, arg := range e.args {
 			v, isNull, err = arg.EvalString(sctx, row)
 			if err != nil {
 				return err
@@ -489,7 +500,7 @@ func (e *groupConcatDistinctOrder) UpdatePartialResult(sctx sessionctx.Context,
 			if isNull {
 				break
 			}
-			p.encodeBytesBuffer = codec.EncodeBytes(p.encodeBytesBuffer, hack.Slice(v))
+			p.encodeBytesBuffer = codec.EncodeBytes(p.encodeBytesBuffer, collators[i].Key(v))
 			buffer.WriteString(v)
 		}
 		if isNull {
diff --git a/executor/analyze_test.go b/executor/analyze_test.go
index 764b1c38ce692..4573394690d36 100644
--- a/executor/analyze_test.go
+++ b/executor/analyze_test.go
@@ -648,6 +648,19 @@ func (s *testSuite1) TestDefaultValForAnalyze(c *C) {
 		"└─IndexRangeScan_5 1.00 cop[tikv] table:t, index:a(a) range:[1,1], keep order:false"))
 }
 
+func (s *testSerialSuite2) TestIssue27429(c *C) {
+	collate.SetNewCollationEnabledForTest(true)
+	defer collate.SetNewCollationEnabledForTest(false)
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t")
+	tk.MustExec("create table test.t(id int, value varchar(20) charset utf8mb4 collate utf8mb4_general_ci, value1 varchar(20) charset utf8mb4 collate utf8mb4_bin)")
+	tk.MustExec("insert into test.t values (1, 'abc', 'abc '),(4, 'Abc', 'abc'),(3,'def', 'def ');")
+
+	tk.MustQuery("select upper(group_concat(distinct value order by 1)) from test.t;").Check(testkit.Rows("ABC,DEF"))
+	tk.MustQuery("select upper(group_concat(distinct value)) from test.t;").Check(testkit.Rows("ABC,DEF"))
+}
+
 func (s *testSerialSuite2) TestIssue20874(c *C) {
 	collate.SetNewCollationEnabledForTest(true)
 	defer collate.SetNewCollationEnabledForTest(false)
diff --git a/expression/constant_propagation.go b/expression/constant_propagation.go
index 7c521d7ea37ae..b383168d5fcf4 100644
--- a/expression/constant_propagation.go
+++ b/expression/constant_propagation.go
@@ -146,7 +146,7 @@ func tryToReplaceCond(ctx sessionctx.Context, src *Column, tgt *Column, cond Exp
 			sf.FuncName.L == ast.If ||
 			sf.FuncName.L == ast.Case ||
 			sf.FuncName.L == ast.NullEQ) {
-		return false, false, cond
+		return false, true, cond
 	}
 	for idx, expr := range sf.GetArgs() {
 		if src.Equal(nil, expr) {
diff --git a/expression/integration_test.go b/expression/integration_test.go
index 24ab9082f2baa..b3a3da314dac0 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -8264,3 +8264,21 @@ func (s *testIntegrationSuite) TestIssue27236(c *C) {
 	row = tk.MustQuery(`select extract(hour_second from c1) from t order by c1;`)
 	row.Check(testkit.Rows("-8385959", "7005959"))
 }
+
+func (s *testIntegrationSuite) TestConstPropNullFunctions(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+	tk.MustExec("use test")
+	tk.MustExec("drop table if exists t1, t2")
+	tk.MustExec("create table t1 (a integer)")
+	tk.MustExec("insert into t1 values (0), (1), (2), (3)")
+	tk.MustExec("create table t2 (a integer, b integer)")
+	tk.MustExec("insert into t2 values (0,1), (1,1), (2,1), (3,1)")
+	tk.MustQuery("select t1.* from t1 left join t2 on t2.a = t1.a where t1.a = ifnull(t2.b, 0)").Check(testkit.Rows("1"))
+
+	tk.MustExec("drop table if exists t1, t2")
+	tk.MustExec("create table t1 (i1 integer, c1 char)")
+	tk.MustExec("insert into t1 values (2, 'a'), (1, 'b'), (3, 'c'), (0, null);")
+	tk.MustExec("create table t2 (i2 integer, c2 char, f2 float)")
+	tk.MustExec("insert into t2 values (0, 'c', null), (1, null, 0.1), (3, 'b', 0.01), (2, 'q', 0.12), (null, 'a', -0.1), (null, null, null)")
+	tk.MustQuery("select * from t2 where t2.i2=((select count(1) from t1 where t1.i1=t2.i2))").Check(testkit.Rows("1 <nil> 0.1"))
+}