sql - MySQL - 获取多个分组行的摘要？

Question

2 个表:所有者和汽车

一个车主可以拥有多辆汽车。汽车可以标记为 usable_offroad、usable_onroad 或两者。 cars 表有 usable offroad 和 usable_onroad 字段，可以设置为 0 或 1(no 或 yes)

考虑以下查询:

SELECT *
FROM owners
LEFT JOIN cars on cars.owner_id = owners.id
GROUP BY owners.id
ORDER BY owners.last_name

我的目标是返回车主列表，以及每个车主是否拥有公路车或 buggy ，或两者兼而有之:

    Last Name    First Name    Has Offroad Car      Has Onroad Car
----------------------------------------------------------------------
    Smith        Todd          Yes                   No
    Smith        Tom           Yes                   Yes
    Test         Sue           No                    Yes
    Thumb        Joe           No                    No
    White        Al            Yes                   No

如何查询？正在考虑使用 ROLLUP，但如果摘要不是附加行而是已经分组的所有者行上的实际字段，我会更喜欢。

Answer 1

使用:

   SELECT DISTINCT
          o.lastname,
          o.firstname,
          CASE WHEN COALESCE(y.num_offroad, 0) > 0 THEN 'yes' ELSE 'no' END AS "Has Offroad Car"
          CASE WHEN COALESCE(x.num_onroad, 0) > 0 THEN 'yes' ELSE 'no' END AS "Has Onroad Car"
     FROM OWNERS o
LEFT JOIN (SELECT c.owner_id,
                  COUNT(*) AS num_onroad
             FROM CARS c
            WHERE c.usable_onroad = 1
         GROUP BY c.owner_id) x ON x.owner_id = o.id
LEFT JOIN (SELECT c.owner_id,
                  COUNT(*) AS num_offroad
             FROM CARS c
            WHERE c.usable_offroad = 1
         GROUP BY c.owner_id) y ON y.owner_id = o.id