SQL SELECT with GROUP BY，指回组

Question

我是一个相对的 SQL 初学者，我不完全确定如何问我在问什么（所以我也不完全确定如何搜索有关它的帮助）。

我想使用SELECT 语句，该语句使用GROUP BY 子句按某个属性分组，同时引用该组选择的行。有没有办法做到这一点？ （我使用的是 FWIW、SQLite3，但我认为这更多是通用 SQL 语法的问题。）

我有一张表population，其中包含有关人员和家庭的数据。每排是一个人，每个人都被分成从一到十几个的家庭。

每个家庭一个人（通常是第一个人，uid）是户主，并设置了head 列。我想GROUP BY 列household_id，并且还选择作为“头”的组中每个成员的uid。有没有办法用一个语句来做到这一切？

这是我所拥有的：

SELECT DISTINCT household_id,
    COUNT(uid) AS members   /* The number of members in each household; it works */,
    (SELECT uid FROM "group" WHERE head == 1) /* Only I don't know how to do this */
    FROM population GROUP BY household_id;

我尝试过这样的事情，但它没有给我想要的东西：

SELECT DISTINCT household_id,
    COUNT(uid) AS members,
    (SELECT uid FROM population WHERE household_id == household_id AND head == 1) /* <-- here */
    FROM population GROUP BY household_id;

但这当然会选择household_id 是household_id 的第一行；即表中的第一行。我如何仅引用此特定组中的household_id 即DISTINCT？

有什么建议吗？提前致谢。

Answer 1

试试看，

SELECT  household_id,
        COUNT(uid) AS members, 
        MAX(CASE WHEN head = 1 THEN uid ELSE NULL END) headID
FROM    population 
GROUP   BY household_id;

• SQLFiddle Demo

Answer 2

你本可以走在正确的轨道上

SELECT DISTINCT household_id,
COUNT(uid) AS members,
(SELECT uid FROM population WHERE household_id == household_id AND head == 1) /* <-- here */
FROM population GROUP BY household_id;

但您需要为 population 表设置别名，因为它在查询中使用了两次。

SELECT DISTINCT household_id,
COUNT(uid) AS members,
(SELECT uid FROM population gr_head WHERE pop.household_id = gr_head.household_id AND head == 1) /* <-- here */
FROM population pop GROUP BY household_id;