mybatisplus怎么三表联查

Mybatis Plus支持使用注解或XML配置实现三表联查。以下是使用注解实现三表联查的示例：

假设有三个表：user、order和order_item，它们的关系是：user和order是一对多的关系，order和order_item是一对多的关系。

1. 在UserMapper接口中添加方法：

@Select("SELECT u.*, o.*, oi.* FROM user u LEFT JOIN order o ON u.id = o.user_id LEFT JOIN order_item oi ON o.id = oi.order_id WHERE u.id = #{userId}")
List<Map<String, Object>> getUserOrderItem(Long userId);

2. 在OrderMapper接口中添加方法：

@Select("SELECT o.*, oi.* FROM order o LEFT JOIN order_item oi ON o.id = oi.order_id WHERE o.user_id = #{userId}")
List<Map<String, Object>> getOrderItem(Long userId);

3. 在OrderItemMapper接口中添加方法：

@Select("SELECT * FROM order_item WHERE order_id = #{orderId}")
List<OrderItem> getByOrderId(Long orderId);

4. 在UserService中调用方法：

public List<User> getUserOrderItem(Long userId) {
    List<Map<String, Object>> result = userMapper.getUserOrderItem(userId);
    Map<Long, User> userMap = new HashMap<>();
    Map<Long, Order> orderMap = new HashMap<>();
    List<OrderItem> orderItemList = new ArrayList<>();

    for (Map<String, Object> map : result) {
        Long userId = Long.valueOf(map.get("id").toString());
        User user = userMap.get(userId);
        if (user == null) {
            user = new User();
            user.setId(userId);
            user.setUsername(map.get("username").toString());
            userMap.put(userId, user);
        }

        Long orderId = Long.valueOf(map.get("o_id").toString());
        Order order = orderMap.get(orderId);
        if (order == null) {
            order = new Order();
            order.setId(orderId);
            order.setUserId(userId);
            order.setOrderNo(map.get("order_no").toString());
            orderMap.put(orderId, order);
        }

        OrderItem orderItem = new OrderItem();
        orderItem.setId(Long.valueOf(map.get("oi_id").toString()));
        orderItem.setOrderId(orderId);
        orderItem.setProductNo(map.get("product_no").toString());
        orderItem.setQuantity(Integer.valueOf(map.get("quantity").toString()));
        orderItemList.add(orderItem);
    }

    List<User> userList = new ArrayList<>(userMap.values());
    for (User user : userList) {
        List<Order> orderList = new ArrayList<>();
        for (Order order : orderMap.values()) {
            if (order.getUserId().equals(user.getId())) {
                List<OrderItem> oiList = new ArrayList<>();
                for (OrderItem oi : orderItemList) {
                    if (oi.getOrderId().equals(order.getId())) {
                        oiList.add(oi);
                    }
                }
                order.setOrderItemList(oiList);
                orderList.add(order);
            }
        }
        user.setOrderList(orderList);
    }

    return userList;
}

此时，调用getUserOrderItem方法可以获得一个User对象，其中包含该用户的所有订单以及每个订单的商品项。