Question

SELECT COUNT(*) as not_returned_devices, contact_email FROM device_rent WHERE rent_end IS NULL GROUP BY contact_email;

SELECT COUNT(*) as returned_devices, contact_email FROM device_rent WHERE rent_end IS NOT NULL GROUP BY contact_email;

我需要的是创建一个包含3列的表：contact_name，返回的设备数和未返回的设备数。基本上，我需要的是在一个表中合并这两个选项，但我卡住了。

Answer 1

只需使用条件聚合：

SELECT contact_email, SUM(rent_end IS NULL) as not_returned_devices,
       SUM(rent_end IS NOT NULL) as returned_devices
FROM device_rent
GROUP BY contact_email;

请注意，这使用了一个MySQL快捷方式，其中布尔表达式被视为“1”表示true，“0”表示false。这是一种方便，并且可以避免编写case语句。那看起来像是：

SELECT contact_email,
       SUM(CASE WHEN rent_end IS NULL THEN 1 ELSE 0
           END) as not_returned_devices,
       COUNT(rent_end) as returned_devices
FROM device_rent
GROUP BY contact_email;

MySQL创建一个包含2个不同选择的表

1 个答案: