mysql - 从按集合分组的值中选择值

Question

我如何制作这个 SQL Fiddle从 GROUP BY 返回的元素子集中获取结果？

表格

CREATE TABLE IF NOT EXISTS `accounts` (
  `master_id` int(3) unsigned NOT NULL,
  `child_id` int(3) unsigned NOT NULL,
  `name` varchar(200) NOT NULL
);

数据

INSERT INTO `accounts` (`master_id`, `child_id`, `name`) VALUES
  ('222', '555', 'child_555'),
  ('111', '111', 'master_111'),
  ('111', '999', 'child_999'),
  ('111', '888', 'child_888'),
  ('222', '222', 'master_222'),
  ('222', '777', 'child_777'),
  ('111', '666', 'child_666');

当前查询

SELECT 
master_id, 
name, 
count(*) as "Total Accounts"
FROM `accounts`
GROUP BY master_id ASC;

结果

+-----------+------------+-------+
| master_id |    name    | Total |
+-----------+------------+-------+
|       111 | master_111 |     4 |
|       222 | child_555  |     3 |
+-----------+------------+-------+

预期

 - Don't count child if child_id == master_id
 - Get correct name

+-----------+------------+-------+
| master_id |    name    | Total |
+-----------+------------+-------+
|       111 | master_111 |     3 |
|       222 | master_222 |     2 |
+-----------+------------+-------+

Answer 1

我认为带有一些条件逻辑的简单聚合是最好的方法:

SELECT master_id,
       MAX(CASE WHEN child_id = master_id THEN name END) as name,
       SUM(child_id <> master_id) as "Total Accounts"
FROM `accounts`
GROUP BY master_id ASC;

Here是这个的 SQL fiddle 。