很長時間的SQL調用JOIN

Question

有4個表，每個表都是一個元組ID(INT),name(VARCHAR)（該ID是自動增量的）。有一張餐桌（芝士漢堡，漢堡，肉宴，BLT，番茄湯），班（漢堡，比薩，三明治，湯），特色（辣，素食，無麩質）和麪（沙拉，薯條，麪包） 。

三多個表存在，使元組Meal.ID,Class.ID，Meal.ID,Characteristics.ID，Meal.ID,Sides.ID使得膳食可以是在一個以上的類，有一個以上的特性，並配有一個以上的側，反之亦然。

我有下面的延遲加載就好了，但我們都知道這並不好。我想要的是餐的名稱，它是同一班的特色，餐廳和其他餐點。

此調用工作就好了：

SELECT Group_concat(DISTINCT m2.meal_name SEPARATOR ',')   AS alternates, 
     Group_concat(DISTINCT c.characteristic_desc SEPARATOR ',') AS 
     characteristics 
FROM meal AS M 
     INNER JOIN mealclass AS MC 
       ON M.meal_id = MC.meal_id 
     LEFT JOIN (SELECT meal_id, 
         class_id 
        FROM drugingredient) AS MC2 
       ON MC2.class_id = MC.class_id 
     LEFT JOIN meals AS M2 
       ON MC2.meal_id = M2.meal_id 
     LEFT JOIN mealchar AS MCh 
       ON MCh.meal_id = M.meal_id 
     INNER JOIN characterisics AS C 
       ON C.characteristic_id = MCh.characteristic_id 
WHERE M.meal_id = :meal_id 
     AND M.meal_id <> M2.meal_id 

但是，當我伸出出來這個，我什麼都沒回：

SELECT Group_concat(DISTINCT m2.meal_name SEPARATOR ',')   AS alternates, 
     Group_concat(DISTINCT c.characteristic_desc SEPARATOR ',') AS 
     characteristics, 
     Group_concat(DISTINCT s.sides_desc SEPARATOR ',')   AS side_orders 
FROM meal AS M 
     INNER JOIN mealclass AS MC 
       ON M.meal_id = MC.meal_id 
     LEFT JOIN (SELECT meal_id, 
         class_id 
        FROM drugingredient) AS MC2 
       ON MC2.class_id = MC.class_id 
     LEFT JOIN meals AS M2 
       ON MC2.meal_id = M2.meal_id 
     LEFT JOIN mealchar AS MCh 
       ON MCh.meal_id = M.meal_id 
     INNER JOIN characterisics AS C 
       ON C.characteristic_id = MCh.characteristic_id 
     INNER JOIN mealsides AS MS 
       ON M.meal_id = MS.meal_id 
     INNER JOIN sides AS S 
       ON S.sides_id = MS.sides_id 
WHERE M.meal_id = :meal_id 
     AND M.meal_id <> M2.meal_id 

任何想法如何解決呼叫，或如何有一個更好的架構模式？或者是一定程度的懶惰加載最好的方式去這裏（：P）？！

Answer 1

有三種可能的事情，你想在這裏。我把它們寫成了一個示範。

create table a (x integer, y integer); 
create table b (v integer, w integer); 
create table c (t integer, u integer); 
insert into a values (1,2); 
insert into a values (3,4); 
insert into a values (5,6); 
insert into b values (1,2); 
insert into b values (5,6); 
insert into c values (1,2); 
insert into c values (3,4); 

這是一樣的，你用：
（A LEFT JOIN B）將C

select * from a left join b on a.x = b.v join c on b.v = c.t; 
x | y | v | w | t | u 
---+---+---+---+---+--- 
1 | 2 | 1 | 2 | 1 | 2 

這將只用左聯接：
（A LEFT JOIN B）LEFT JOIN ç

select * from a left join b on a.x = b.v left join c on b.v = c.t; 
x | y | v | w | t | u 
---+---+---+---+---+--- 
1 | 2 | 1 | 2 | 1 | 2 
3 | 4 | | | | 
5 | 6 | 5 | 6 | | 

我認爲這是你瞄準： 一個LEFT JOIN（B JOIN C）

select * from a left join (b join c on b.v = c.t) on a.x = b.v; 
x | y | v | w | t | u 
---+---+---+---+---+--- 
1 | 2 | 1 | 2 | 1 | 2 
3 | 4 | | | | 
5 | 6 | | | | 

希望這有助於