如何結合兩個SQL查詢？

Question

我有一個庫存表，我想創建一個報告，顯示訂購商品的頻率。

“股票”表：

item_id |  pcs | operation
apples  |  100 | order
oranges |   50 | order
apples  | -100 | delivery
pears   |  100 | order
oranges |  -40 | delivery
apples  |   50 | order
apples  |   50 | delivery

基本上我需要將這兩個查詢連接在一起。

打印庫存余額的查詢：

SELECT stock.item_id, Sum(stock.pcs) AS stock_balance
FROM stock
GROUP BY stock.item_id;

打印銷售統計信息的查詢

SELECT stock.item_id, Sum(stock.pcs) AS pcs_ordered, Count(stock.item_id) AS number_of_orders
FROM stock
GROUP BY stock.item_id, stock.operation
HAVING stock.operation="order";

我認為某種JOIN可以完成這項工作，但我不知道如何將查詢粘合在一起。

期望的輸出：

item_id | stock_balance | pcs_ordered | number_of_orders
apples  |             0 |         150 |                2
oranges |            10 |          50 |                1
pears   |           100 |         100 |                1

這只是一個例子。 也許，我需要添加更多條件，因為有更多列。 是否存在將多個查詢組合在一起的通用技術？

Answer 1

SELECT a.item_id, a.stock_balance, b.pcs_ordered, b.number_of_orders
FROM
    (SELECT stock.item_id, Sum(stock.pcs) AS stock_balance 
    FROM stock 
    GROUP BY stock.item_id) a
LEFT OUTER JOIN
    (SELECT stock.item_id, Sum(stock.pcs) AS pcs_ordered, 
            Count(stock.item_id) AS number_of_orders 
    FROM stock
    WHERE stock.operation = "order"
    GROUP BY stock.item_id) b
ON a.item_id = b.item_id

Answer 2

這應該做到這一點

SELECT
    stock.item_id, 
    Sum(stock.pcs) AS stock_balance,
    pcs_ordered,
    number_of_orders
FROM stock LEFT OUTER JOIN (
    SELECT stock.item_id,
    SUM(stock.pcs) AS pcs_ordered,
    COUNT(stock.item_id) AS number_of_orders
    FROM stock
    WHERE stock.operation ='order'
    GROUP BY stock.item_id
    ) s2 ON stock.item_id = s2.item_id
GROUP BY 
    stock.item_id,
    pcs_ordered,
    number_of_orders