diff --git a/executor/index_lookup_join.go b/executor/index_lookup_join.go
index 270036ac91b12..f57079d2972b1 100644
--- a/executor/index_lookup_join.go
+++ b/executor/index_lookup_join.go
@@ -532,6 +532,10 @@ func (iw *innerWorker) buildLookUpMap(task *lookUpJoinTask) error {
 		chk := task.innerResult.GetChunk(i)
 		for j := 0; j < chk.NumRows(); j++ {
 			innerRow := chk.GetRow(j)
+			if iw.hasNullInJoinKey(innerRow) {
+				continue
+			}
+
 			keyBuf = keyBuf[:0]
 			for _, keyCol := range iw.keyCols {
 				d := innerRow.GetDatum(keyCol, iw.rowTypes[keyCol])
@@ -549,6 +553,15 @@ func (iw *innerWorker) buildLookUpMap(task *lookUpJoinTask) error {
 	return nil
 }
 
+func (iw *innerWorker) hasNullInJoinKey(row chunk.Row) bool {
+	for _, ordinal := range iw.keyCols {
+		if row.IsNull(ordinal) {
+			return true
+		}
+	}
+	return false
+}
+
 // Close implements the Executor interface.
 func (e *IndexLookUpJoin) Close() error {
 	if e.cancelFunc != nil {
diff --git a/executor/join_test.go b/executor/join_test.go
index 9a697e7633f33..0b36b2b9ad2ec 100644
--- a/executor/join_test.go
+++ b/executor/join_test.go
@@ -845,6 +845,14 @@ func (s *testSuite) TestIndexLookupJoin(c *C) {
 	tk.MustExec("CREATE TABLE t(a BIGINT PRIMARY KEY, b BIGINT);")
 	tk.MustExec("INSERT INTO t VALUES(1, 2);")
 	tk.MustQuery("SELECT /*+ TIDB_INLJ(t1, t2) */ * FROM t t1 JOIN t t2 ON t1.a=t2.a UNION ALL SELECT /*+ TIDB_INLJ(t1, t2) */ * FROM t t1 JOIN t t2 ON t1.a=t2.a;").Check(testkit.Rows("1 2 1 2", "1 2 1 2"))
+
+	tk.MustExec(`drop table if exists t;`)
+	tk.MustExec(`create table t(a decimal(6,2), index idx(a));`)
+	tk.MustExec(`insert into t values(1.01), (2.02), (NULL);`)
+	tk.MustQuery(`select /*+ TIDB_INLJ(t1) */ t1.a from t t1 join t t2 on t1.a=t2.a order by t1.a;`).Check(testkit.Rows(
+		`1.01`,
+		`2.02`,
+	))
 }
 
 func (s *testSuite) TestMergejoinOrder(c *C) {
@@ -882,4 +890,12 @@ func (s *testSuite) TestMergejoinOrder(c *C) {
 		`2 1 2 1`,
 		`2 2 2 2`,
 	))
+
+	tk.MustExec(`drop table if exists t;`)
+	tk.MustExec(`create table t(a decimal(6,2), index idx(a));`)
+	tk.MustExec(`insert into t values(1.01), (2.02), (NULL);`)
+	tk.MustQuery(`select /*+ TIDB_SMJ(t1) */ t1.a from t t1 join t t2 on t1.a=t2.a order by t1.a;`).Check(testkit.Rows(
+		`1.01`,
+		`2.02`,
+	))
 }
diff --git a/executor/merge_join.go b/executor/merge_join.go
index b869ce2449b1f..eaf39429f2dca 100644
--- a/executor/merge_join.go
+++ b/executor/merge_join.go
@@ -129,23 +129,39 @@ func (t *mergeJoinInnerTable) rowsWithSameKey() ([]chunk.Row, error) {
 }
 
 func (t *mergeJoinInnerTable) nextRow() (chunk.Row, error) {
-	if t.curRow == t.curIter.End() {
-		t.reallocReaderResult()
-		oldMemUsage := t.curResult.MemoryUsage()
-		err := t.reader.Next(t.ctx, t.curResult)
-		// error happens or no more data.
-		if err != nil || t.curResult.NumRows() == 0 {
-			t.curRow = t.curIter.End()
-			return t.curRow, errors.Trace(err)
+	for {
+		if t.curRow == t.curIter.End() {
+			t.reallocReaderResult()
+			oldMemUsage := t.curResult.MemoryUsage()
+			err := t.reader.Next(t.ctx, t.curResult)
+			// error happens or no more data.
+			if err != nil || t.curResult.NumRows() == 0 {
+				t.curRow = t.curIter.End()
+				return t.curRow, errors.Trace(err)
+			}
+			newMemUsage := t.curResult.MemoryUsage()
+			t.memTracker.Consume(newMemUsage - oldMemUsage)
+			t.curRow = t.curIter.Begin()
+		}
+
+		result := t.curRow
+		t.curResultInUse = true
+		t.curRow = t.curIter.Next()
+
+		if !t.hasNullInJoinKey(result) {
+			return result, nil
+		}
+	}
+}
+
+func (t *mergeJoinInnerTable) hasNullInJoinKey(row chunk.Row) bool {
+	for _, col := range t.joinKeys {
+		ordinal := col.Index
+		if row.IsNull(ordinal) {
+			return true
 		}
-		newMemUsage := t.curResult.MemoryUsage()
-		t.memTracker.Consume(newMemUsage - oldMemUsage)
-		t.curRow = t.curIter.Begin()
 	}
-	result := t.curRow
-	t.curResultInUse = true
-	t.curRow = t.curIter.Next()
-	return result, nil
+	return false
 }
 
 // reallocReaderResult resets "t.curResult" to an empty Chunk to buffer the result of "t.reader".