diff --git a/src/main/kotlin/com/cubetiqs/graphql/demo/mutation/UserMutation.kt b/src/main/kotlin/com/cubetiqs/graphql/demo/mutation/UserMutation.kt
index d340a7d..32ce2a7 100644
--- a/src/main/kotlin/com/cubetiqs/graphql/demo/mutation/UserMutation.kt
+++ b/src/main/kotlin/com/cubetiqs/graphql/demo/mutation/UserMutation.kt
@@ -16,6 +16,8 @@ class UserMutation @Autowired constructor(
 ) : GraphQLMutationResolver {
     @Transactional(propagation = Propagation.REQUIRES_NEW)
     fun createUser(input: UserInput): User {
+        if (userRepository.existsAllByUsername(input.username ?: "")) throw Exception("Username has been already existed!")
+
         val user = UserMapper.fromInputToUser(input)
         return userRepository.save(user)
     }
diff --git a/src/main/kotlin/com/cubetiqs/graphql/demo/repository/UserRepository.kt b/src/main/kotlin/com/cubetiqs/graphql/demo/repository/UserRepository.kt
index 80678a8..a1800d4 100644
--- a/src/main/kotlin/com/cubetiqs/graphql/demo/repository/UserRepository.kt
+++ b/src/main/kotlin/com/cubetiqs/graphql/demo/repository/UserRepository.kt
@@ -11,4 +11,7 @@ import org.springframework.stereotype.Repository
 interface UserRepository : JpaRepository<User, Long> {
     @Query("select u from User u where u.enabled = true")
     fun queryAllByEnabledIsTrue(pageable: Pageable): Page<User>
+
+    @Query("select (count(u) > 0) from User u where u.username = ?1")
+    fun existsAllByUsername(username: String): Boolean
 }
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