比較Postgres中的JSON數組

Question

我有一個表格表示不同時刻購物車中的物品。購物車商品以JSON格式存儲，其中一列的值爲before，另一列的值爲after。

我的任務是比較購物車和識別：添加的項目，刪除的項目和價格/數量的變化。

我知道我可以使用json_array_elements和一個交叉連接來解開JSON blob，從JSON值創建一個項目表。但我想不出一種比較購物車物品的有效方法。我可以在SQL中執行此操作嗎？

這裏是一個小例子SQL Fiddle：

CREATE TABLE t (
    id  INT, 
    before JSON, 
    after JSON 
); 

INSERT INTO t VALUES (
    1, 
    '[{"category":"item","id":"1","price":8,"quantity":1},{"category":"item","id":"2","price":20,"quantity":1},{"category":"item","id":"3","price":3,"quantity":1}]', 
    '[{"category":"item","id":"2","price":40,"quantity":1},{"category":"item","id":"3","price":3,"quantity":1},{"category":"item","id":"4","price":2,"quantity":1}]' 
); 

這種昂貴查詢不解決我的問題：

select 
    id, 
    b.value->>'id' as id_before, 
    b.value->>'category' as category_before, 
    b.value->>'price' as price_before, 
    b.value->>'quantity' as quantity_before, 
    a.value->>'id' as id_after, 
    a.value->>'category' as category_after, 
    a.value->>'price' as price_after, 
    a.value->>'quantity' as quantity_after 
from t 
CROSS JOIN json_array_elements(before) b 
CROSS JOIN json_array_elements(after) a; 

Answer 1

可以使用除了所有讓你的兩個JSON之間的差異，通過這可以得到哪個項目被添加和刪除。 爲了找到差異我的查詢將是：

select 
value->>'id', 
value->>'category', 
value->>'price', 
value->>'quantity' 
FROM 
    json_array_elements(before) 
EXCEPT ALL 
select 
value->>'id', 
value->>'category', 
value->>'price', 
value->>'quantity' 
FROM 
    json_array_elements(after)