diff --git a/cmd/main.go b/cmd/main.go
new file mode 100644
index 0000000..9864979
--- /dev/null
+++ b/cmd/main.go
@@ -0,0 +1,73 @@
+package main
+
+import (
+	"database/sql"
+	"fmt"
+
+	"github.com/winebarrel/dbtyp"
+	_ "modernc.org/sqlite"
+)
+
+type AliceDB struct{}
+type BobDB struct{}
+
+type MyDB interface {
+	AliceDB | BobDB
+}
+
+func main() {
+	aliceDB := openDB[AliceDB]()
+	bobDB := openDB[BobDB]()
+
+	// bob = alice // COMPILE ERROR!
+
+	createTable(aliceDB, "foo")
+	createTable(bobDB, "bar")
+
+	procForAlice(aliceDB.ExecQueryer())
+	// procForAlice(bob.ExecQueryer()) // COMPILE ERROR!
+
+	procForBob(bobDB.Queryer())
+	// procForBob(alice.Queryer()) // COMPILE ERROR!
+}
+
+func openDB[T MyDB]() *dbtyp.DB[T] {
+	db, err := dbtyp.New2[T](sql.Open("sqlite", "file::memory:"))
+
+	if err != nil {
+		panic(err)
+	}
+
+	return db
+}
+
+func createTable[T MyDB](db *dbtyp.DB[T], name string) {
+	_, err := db.Exec("create table " + name + " (id int)")
+
+	if err != nil {
+		panic(err)
+	}
+}
+
+func procForAlice(eq *dbtyp.ExecQueryer[AliceDB]) {
+	_, err := eq.Exec("insert into foo values (1)")
+	if err != nil {
+		panic(err)
+	}
+
+	var n int
+	err = eq.QueryRow("select count(*) from foo").Scan(&n)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println("foo rows count:", n)
+}
+
+func procForBob(q *dbtyp.Queryer[BobDB]) {
+	var n int
+	err := q.QueryRow("select count(*) from bar").Scan(&n)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println("bar rows count:", n)
+}
diff --git a/db.go b/db.go
index 9d8db20..37fc665 100644
--- a/db.go
+++ b/db.go
@@ -7,10 +7,8 @@ import (
 	"github.com/winebarrel/dbtyp/iface"
 )
 
-var _ iface.DB = &DB[struct{}]{}
-
 type DB[T any] struct {
-	*sql.DB
+	iface.DB
 }
 
 // Type converter
diff --git a/stmt.go b/stmt.go
index eefc4ef..f53205e 100644
--- a/stmt.go
+++ b/stmt.go
@@ -1,13 +1,9 @@
 package dbtyp
 
 import (
-	"database/sql"
-
 	"github.com/winebarrel/dbtyp/iface"
 )
 
-var _ iface.Stmt = &Stmt[struct{}]{}
-
 type Stmt[T any] struct {
-	*sql.Stmt
+	iface.Stmt
 }
diff --git a/tx.go b/tx.go
index c68bf84..4e9a433 100644
--- a/tx.go
+++ b/tx.go
@@ -2,15 +2,12 @@ package dbtyp
 
 import (
 	"context"
-	"database/sql"
 
 	"github.com/winebarrel/dbtyp/iface"
 )
 
-var _ iface.Tx = &Tx[struct{}]{}
-
 type Tx[T any] struct {
-	*sql.Tx
+	iface.Tx
 }
 
 // Type converter