在下面的示例中，确定何时对日期使用date_trunc函数
我试过这个密码

SELECT
  -- Select eatery and calculate total cost
  eatery,
  DATE_TRUNC('month', stocking_date) :: DATE AS delivr_month,
  sum(meal_cost) :: FLOAT AS cost
FROM meals
JOIN stock ON meals.meal_id = stock.meal_id
-- Keep only the records after October 2018
WHERE stocking_date>'2018-10-01'
GROUP BY eatery, delivr_month
ORDER BY eatery, delivr_month;

但正确答案如下，我不明白为什么我们需要在这里使用date_trunc函数，因为查询可以运行没有它

SELECT
  -- Select eatery and calculate total cost
  eatery,
  DATE_TRUNC('month', stocking_date) :: DATE AS delivr_month,
  SUM(meal_cost * stocked_quantity) :: FLOAT AS cost
FROM meals
JOIN stock ON meals.meal_id = stock.meal_id
-- Keep only the records after October 2018
WHERE DATE_TRUNC('month', stocking_date) > '2018-10-01'
GROUP BY eatery, delivr_month
ORDER BY eatery, delivr_month;