比较Postgres中的JSON数组

Question

我有一个表格表示不同时刻购物车中的物品。购物车商品以JSON格式存储，其中一列的值为before，另一列的值为after。

我的任务是比较购物车和识别：添加的项目，删除的项目和价格/数量的变化。

我知道我可以使用json_array_elements和一个交叉连接来解开JSON blob，从JSON值创建一个项目表。但我想不出一种比较购物车物品的有效方法。我可以在SQL中执行此操作吗？

这里是一个小例子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}]' 
); 

这种昂贵查询不解决我的问题：

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

可以使用除了所有让你的两个JSON之间的差异，通过这可以得到哪个项目被添加和删除。 为了找到差异我的查询将是：

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)