Mysql - 「group by group_concat（）」的可能解決方案？

Question

是否可以通過聚合函數的結果對結果進行分組，而不將結果嵌套在自身上？

"SELECT b.pizza, GROUP_CONCAT(t.topping_names SEPARATOR ',') AS topping_string FROM bascket AS b 
LEFT JOIN toppings AS t ON t.pizza = b.pizza 
GROUP BY pizza, topping_string " 

上面的查詢將返回由他們所選擇的topping_string分組的所有比薩餅（應返回獨特的比薩澆頭+）。但是因爲group_concat是在分組之後完成的，所以這是無效的。因此，我認爲唯一的選擇將是換行查詢，並使其實際分組的子查詢：

SELECT sub.* FROM ( 
SELECT b.pizza, GROUP_CONCAT(t.topping_names SEPARATOR ',') AS topping_string FROM bascket AS b 
    LEFT JOIN toppings AS t ON t.pizza = b.pizza 
) AS sub 
GROUP BY sub.pizza, sub.topping_string 

這是正確的方式做到這一點？有另外一種選擇嗎？

Answer 1

是的，如果你正在做兩個聚合，這是正確的做法，沒有辦法在子查詢中執行第一個聚合。

但是，你沒有任何的聚合，因此您可以通過使用DISTINCT強制唯一性：

SELECT DISTINCT 
     b.pizza, 
     GROUP_CONCAT(t.topping_names SEPARATOR ',') AS topping_string 
FROM bascket AS b 
    LEFT JOIN toppings AS t 
     ON t.pizza = b.pizza 
GROUP BY b.pizza; 

注：如果您確實需要執行聚合，則您在子查詢中缺少一個組，應該是：

SELECT sub.*, COUNT(*) AS Quantity FROM ( 
SELECT b.pizza, GROUP_CONCAT(t.topping_names SEPARATOR ',') AS topping_string 
FROM bascket AS b 
    LEFT JOIN toppings AS t ON t.pizza = b.pizza 
GROUP BY b.pizza 
) AS sub 
GROUP BY sub.pizza, sub.topping_string