Update data in Multiple table - mysql

Question

Three table as follow
mysql> select * from food;
+--------+------+-------+
| foodid | name | price |
+--------+------+-------+
| 1 | 雞 | 100 |
| 2 | 鴨 | 200 |
| 3 | 魚 | 300 |
| 4 | 肉 | 400 |
+--------+------+-------+
4 rows in set
mysql> select * from drink;
+---------+------+-------+
| drinkid | name | price |
+---------+------+-------+
| 1 | 紅茶 | 50 |
| 2 | 綠茶 | 100 |
| 3 | 奶茶 | 150 |
+---------+------+-------+
3 rows in set
mysql> select * from order_table;
+----+-----------+--------+---------+------------+-------------+-------------+
| id | user_name | foodid | drinkid | food_count | drink_count | total_price |
+----+-----------+--------+---------+------------+-------------+-------------+
| 2 | 小明 | 3 | 2 | 2 | 2 | 0 |
| 3 | 小華 | 1 | 1 | 1 | 8 | 0 |
| 4 | 小英 | 1 | 3 | 3 | 3 | 0 |
| 6 | 小a | 2 | 1 | 4 | 6 | 0 |
| 7 | 小b | 2 | 2 | 5 | 4 | 0 |
| 8 | 小c | 2 | 3 | 6 | 10 | 0 |
| 9 | 大A | 3 | 1 | 9 | 8 | 0 |
| 10 | 大B | 3 | 2 | 5 | 4 | 0 |
| 11 | 大C | 3 | 3 | 10 | 3 | 0 |
+----+-----------+--------+---------+------------+-------------+-------------+
foodid in order_table is link to foodid in food table,
drinkid in order_table is link to drinkid in drink table,
Now, I want to calculate total price,
Total_price =
order_table.foodid(food.price in food table) * order_table.food_count +
order_table.drinkid(drink.price in drink table) * order_table.drink_count;
So, let me knowlege the command to update total price
thx a lot.

Answer 1

The reason why I used LEFT JOIN on the following query is because I assumed that some orders may only contain drinks or foods.
UPDATE order_table a
LEFT JOIN food b
ON a.foodid = b.foodID
LEFT JOIN drink c
ON a.drinkID = c.drinkID
SET a.total_price = (a.food_count * COALESCE(b.price, 0) +
a.drink_count * COALESCE(c.price, 0))
SQLFiddle Demo
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins

Answer 2

Something like this should be close:
SELECT COALESCE(F.Price,0)*OT.Food_Count+COALESCE(D.Price,0)*OT.Drink_Count Total_price
FROM Order_Table OT
LEFT JOIN Food F ON OT.FoodId = F.FoodId
LEFT JOIN Drink D ON OT.DrinkId = D.DrinkId
And to actually update that column:
UPDATE Order_Table OT
LEFT JOIN Food F ON OT.FoodId = F.FoodId
LEFT JOIN Drink D ON OT.DrinkId = D.DrinkId
SET OT.Total_Price = COALESCE(F.Price,0)*OT.Food_Count+COALESCE(D.Price,0)*OT.Drink_Count