Question

我正在尝试查找在SQL中收到的消息的成功率。因此，我创建一个带有消息ID和操作（已发送或已接收）的表，然后计算否。收到的消息数/总数消息。这是我的代码：

CREATE TABLE msg (id INTEGER, action TEXT);
INSERT INTO msg VALUES (1, 'sent');
INSERT INTO msg VALUES (1, 'received');
INSERT INTO msg VALUES (2, 'sent');
INSERT INTO msg VALUES (2, 'received');
INSERT INTO msg VALUES (3, 'sent');
INSERT INTO msg VALUES (4, 'sent');
INSERT INTO msg VALUES (5, 'sent');

SELECT SUM(CASE WHEN action = 'received' THEN 1 ELSE 0 END)/COUNT( DISTINCT id) AS success_rate FROM msg;

这给我0作为输出。为什么？

Answer 1

因为您的数据库进行整数除法。简单添加一个小数点：

SELECT SUM(CASE WHEN action = 'received' THEN 1.0 ELSE 0 END) / COUNT(DISTINCT id) AS success_rate
FROM msg;

我可能想除以发送的数字-以防万一：

SELECT (SUM(CASE WHEN action = 'received' THEN 1.0 ELSE 0 END) /
        SUM(CASE WHEN action = 'sent' THEN 1.0 END)
       ) AS success_rate
FROM msg;

或在两种情况下都使用唯一帐户：

SELECT (COUNT(DISTINCT CASE WHEN action = 'received' THEN id END) /
        COUNT(DISTINCT id)
       ) AS success_rate
FROM msg;

如果id是唯一的（对我来说似乎很合理），则可以简化为：

SELECT AVG(CASE WHEN action = 'received' THEN 1.0 ELSE 0 END) AS success_rate
FROM msg;

为什么INTEGER / INTEGER没有在SQL中给我答案？

1 个答案: