如何对一行进行分组和计数？

Question

为了简化，我有树表：products、products-vs-orders、orders

1. products 字段：'ProductID ', 'Name', 'isGratis', ...
2. products-vs-orders 字段：'ProductID'、'OrderID'
3. orders 字段：'OrderID'、'Title '，...

实际上，我有一个这样的查询：

SELECT orders.OrderID, orders.Title, COUNT(`products`.`isGratis`) AS "Quantity", `products`.`isGratis`
FROM `orders`, `products-vs-orders`, `products`
WHERE `orders`.`OrderID` = `products-vs-orders`.`OrderID` AND `products-vs-orders`.`ProductID` = `products`.`ProductID`
GROUP BY `products`.`PackID`, `products`.`isGratis`

此查询有效并返回结果：

OrderID,    Title,      Quantity,   isGratis
1            My Order      20           0
1            My Order      3            1
2            An other      8            0
2            An other      1            1

如何将产品“免费”和“付费”的计数检索到单独的列中？

OrderID,    Title,      Qt Paid,        Qt Gratis
1            My Order       20              3
2            An other       8               1

Answer 1

试试这个：

SELECT
    orders.OrderID,
    orders.Title, 
    COUNT(orders.OrderId) - SUM(`products`.`isGratis`) AS "Qt Paid", 
    SUM(`products`.`isGratis`) AS "Qt Gratis" 
WHERE `orders`.`OrderID` = `products-vs-orders`.`OrderID`
  AND `products-vs-orders`.`ProductID` = `products`.`ProductID`
GROUP BY `products`.`PackID`

Answer 2

SUM(products.isGratis) 取决于数据库在内部将布尔值表示为单个数字位的事实，因此 false = 0 且 true = 1。ALL

中的情况可能并非如此数据库实现。因此，对布尔字段进行 SUM 可能会导致依赖于实现的行为。

在求和之前将布尔值转换为实际的 0 和 1 值应该更合适：

SELECT orders.OrderID, orders.Title, 
  SUM(CASE WHEN products.isGratis THEN 0 ELSE 1 END) AS "Qt Paid",
  SUM(CASE WHEN products.isGratis THEN 1 ELSE 0 END) AS "Qt Gratis"
FROM orders INNER JOIN `products-vs-orders` ON (orders.OrderID = `products-vs-orders`.OrderID)
  INNER JOIN products ON (`products-vs-orders`.ProductID = products.ProductID)
GROUP BY orders.OrderID, orders.Title

Answer 3

select orderid,title,sum(if(isgratis=0,quantity,0)) as paid,sum(if(isgratis=1,quantity,0)) as gratis from ...