fix null select yielding incorrect results

Author: elianddb

enginetest/queries/script_queries.go

@@ -121,6 +121,60 @@ type ScriptTestAssertion struct {
 // Unlike other engine tests, ScriptTests must be self-contained. No other tables are created outside the definition of
 // the tests.
 var ScriptTests = []ScriptTest{
+	{
+		// https://github.com/dolthub/go-mysql-server/issues/3216
+		Name: "UNION ALL with BLOB columns",
+		SetUpScript: []string{
+			"CREATE TABLE a(name VARCHAR(255), data BLOB)",
+			"CREATE TABLE b(name VARCHAR(255), data BLOB)",
+			"INSERT INTO a VALUES ('a-data', UNHEX('deadbeef'))",
+			"INSERT INTO b VALUES ('b-nodata', NULL)",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "SELECT name, data FROM a UNION ALL SELECT name, data FROM b",
+				Expected: []sql.Row{
+					{"a-data", []byte{0xde, 0xad, 0xbe, 0xef}},
+					{"b-nodata", nil},
+				},
+			},
+			{
+				Query: "SELECT name, HEX(data) as data_hex FROM a UNION ALL SELECT name, HEX(data) as data_hex FROM b",
+				Expected: []sql.Row{
+					{"a-data", "DEADBEEF"},
+					{"b-nodata", nil},
+				},
+			},
+			{
+				Query: "SELECT name, data FROM a UNION ALL SELECT name, NULL FROM b",
+				Expected: []sql.Row{
+					{"a-data", []byte{0xde, 0xad, 0xbe, 0xef}},
+					{"b-nodata", nil},
+				},
+			},
+			{
+				Query: "SELECT name, HEX(data) as data_hex FROM a UNION ALL SELECT name, HEX(NULL) as data_hex FROM b",
+				Expected: []sql.Row{
+					{"a-data", "DEADBEEF"},
+					{"b-nodata", nil},
+				},
+			},
+			{
+				Query: "SELECT name, data FROM a UNION ALL SELECT name, UNHEX('') FROM b",
+				Expected: []sql.Row{
+					{"a-data", []byte{0xde, 0xad, 0xbe, 0xef}},
+					{"b-nodata", []byte{}},
+				},
+			},
+			{
+				Query: "SELECT name, HEX(data) as data_hex FROM a UNION ALL SELECT name, HEX(UNHEX('')) as data_hex FROM b",
+				Expected: []sql.Row{
+					{"a-data", "DEADBEEF"},
+					{"b-nodata", ""},
+				},
+			},
+		},
+	},
 	{
 		// https://github.com/dolthub/dolt/issues/9836
 		Skip: true,

sql/expression/convert.go

@@ -120,6 +120,10 @@ func GetConvertToType(l, r sql.Type) string {
 	}
 
 	if !types.IsNumber(l) || !types.IsNumber(r) {
+		// Special handling for BLOB types - preserve binary data
+		if types.IsBlobType(l) || types.IsBlobType(r) {
+			return ConvertToBinary
+		}
 		return ConvertToChar
 	}