SQL Server：多行的 SUM() 包括 where 子句

使用SQL Server的SUM()函数是计算某一列的总和的常用方法。通常情况下，SUM()函数是用于计算单行的总和。然而，有时我们需要计算多行的总和，并且还需要在计算总和之前应用WHERE子句来筛选出特定的行。在本文中，我们将探讨如何在SQL Server中实现这一目标。

sql
CREATE TABLE sales (
    order_id INT,
    amount DECIMAL(10, 2),
    order_date DATE
);
INSERT INTO sales VALUES (1, 100.00, '2021-01-01');
INSERT INTO sales VALUES (2, 200.00, '2021-01-02');
INSERT INTO sales VALUES (3, 300.00, '2021-01-03');
INSERT INTO sales VALUES (4, 400.00, '2021-01-04');
INSERT INTO sales VALUES (5, 500.00, '2021-01-05');

sql
SELECT SUM(amount) AS total_sales
FROM sales
WHERE order_date BETWEEN '2021-01-01' AND '2021-01-03';

sql
SELECT order_id, amount, order_date
FROM sales
WHERE amount >= 300.00;

sql
SELECT SUM(amount) AS total_sales
FROM sales
WHERE order_date BETWEEN '2021-01-01' AND '2021-01-03'
AND amount >= 300.00;