diff --git a/expression/builtin_cast.go b/expression/builtin_cast.go
index 819912db67a5d..ff556cfc09a3d 100644
--- a/expression/builtin_cast.go
+++ b/expression/builtin_cast.go
@@ -461,9 +461,13 @@ func (b *builtinCastIntAsRealSig) evalReal(row chunk.Row) (res float64, isNull b
 	if isNull || err != nil {
 		return res, isNull, err
 	}
-	if !mysql.HasUnsignedFlag(b.tp.Flag) && !mysql.HasUnsignedFlag(b.args[0].GetType().Flag) {
+	if unsignedArgs0 := mysql.HasUnsignedFlag(b.args[0].GetType().Flag); !mysql.HasUnsignedFlag(b.tp.Flag) && !unsignedArgs0 {
 		res = float64(val)
-	} else if b.inUnion && val < 0 {
+	} else if b.inUnion && !unsignedArgs0 && val < 0 {
+		// Round up to 0 if the value is negative but the expression eval type is unsigned in `UNION` statement
+		// NOTE: the following expressions are equal (so choose the more efficient one):
+		// `b.inUnion && mysql.HasUnsignedFlag(b.tp.Flag) && !unsignedArgs0 && val < 0`
+		// `b.inUnion && !unsignedArgs0 && val < 0`
 		res = 0
 	} else {
 		// recall that, int to float is different from uint to float
@@ -487,9 +491,14 @@ func (b *builtinCastIntAsDecimalSig) evalDecimal(row chunk.Row) (res *types.MyDe
 	if isNull || err != nil {
 		return res, isNull, err
 	}
-	if !mysql.HasUnsignedFlag(b.tp.Flag) && !mysql.HasUnsignedFlag(b.args[0].GetType().Flag) {
+
+	if unsignedArgs0 := mysql.HasUnsignedFlag(b.args[0].GetType().Flag); !mysql.HasUnsignedFlag(b.tp.Flag) && !unsignedArgs0 {
 		res = types.NewDecFromInt(val)
-	} else if b.inUnion && val < 0 {
+		// Round up to 0 if the value is negative but the expression eval type is unsigned in `UNION` statement
+		// NOTE: the following expressions are equal (so choose the more efficient one):
+		// `b.inUnion && mysql.HasUnsignedFlag(b.tp.Flag) && !unsignedArgs0 && val < 0`
+		// `b.inUnion && !unsignedArgs0 && val < 0`
+	} else if b.inUnion && !unsignedArgs0 && val < 0 {
 		res = &types.MyDecimal{}
 	} else {
 		res = types.NewDecFromUint(uint64(val))
diff --git a/expression/builtin_cast_vec.go b/expression/builtin_cast_vec.go
index 81d733e627328..16b452415abc4 100644
--- a/expression/builtin_cast_vec.go
+++ b/expression/builtin_cast_vec.go
@@ -111,7 +111,11 @@ func (b *builtinCastIntAsRealSig) vecEvalReal(input *chunk.Chunk, result *chunk.
 		}
 		if !hasUnsignedFlag0 && !hasUnsignedFlag1 {
 			rs[i] = float64(i64s[i])
-		} else if b.inUnion && i64s[i] < 0 {
+		} else if b.inUnion && !hasUnsignedFlag1 && i64s[i] < 0 {
+			// Round up to 0 if the value is negative but the expression eval type is unsigned in `UNION` statement
+			// NOTE: the following expressions are equal (so choose the more efficient one):
+			// `b.inUnion && hasUnsignedFlag0 && !hasUnsignedFlag1 && i64s[i] < 0`
+			// `b.inUnion && !hasUnsignedFlag1 && i64s[i] < 0`
 			rs[i] = 0
 		} else {
 			// recall that, int to float is different from uint to float
diff --git a/expression/integration_test.go b/expression/integration_test.go
index 642dbc914e88d..78c2dc33f5223 100755
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -2308,6 +2308,22 @@ func (s *testIntegrationSuite2) TestBuiltin(c *C) {
 	result.Check(testkit.Rows("9223372036854775808 9223372036854775808", "9223372036854775808 9223372036854775808"))
 	tk.MustExec(`drop table tb5;`)
 
+	// test builtinCastIntAsDecimalSig
+	tk.MustExec(`drop table if exists tb5`)
+	tk.MustExec(`create table tb5 (a decimal(65), b bigint(64) unsigned);`)
+	tk.MustExec(`insert into tb5 (a, b) values (9223372036854775808, 9223372036854775808);`)
+	result = tk.MustQuery(`select cast(b as decimal(64)) from tb5 union all select b from tb5;`)
+	result.Check(testkit.Rows("9223372036854775808", "9223372036854775808"))
+	tk.MustExec(`drop table tb5`)
+
+	// test builtinCastIntAsRealSig
+	tk.MustExec(`drop table if exists tb5`)
+	tk.MustExec(`create table tb5 (a bigint(64) unsigned, b double(64, 10));`)
+	tk.MustExec(`insert into tb5 (a, b) values (9223372036854775808, 9223372036854775808);`)
+	result = tk.MustQuery(`select a from tb5 where a = b union all select b from tb5;`)
+	result.Check(testkit.Rows("9223372036854776000", "9223372036854776000"))
+	tk.MustExec(`drop table tb5`)
+
 	// Test corner cases of cast string as datetime
 	result = tk.MustQuery(`select cast("170102034" as datetime);`)
 	result.Check(testkit.Rows("2017-01-02 03:04:00"))