Question

您有一个表，您希望计算两个不同表中的项目数。在这个例子中，我使用了建筑物，男人和女人

DROP TABLE IF EXISTS building;
DROP TABLE IF EXISTS men;
DROP TABLE IF EXISTS women;
CREATE TABLE building(name VARCHAR(255));
CREATE TABLE men(building VARCHAR(255), name VARCHAR(255));
CREATE TABLE women(building VARCHAR(255), name VARCHAR(255));
INSERT INTO building VALUES('building1');
INSERT INTO building VALUES('building2');
INSERT INTO building VALUES('building3');
INSERT INTO men VALUES('building1', 'andy');
INSERT INTO men VALUES('building1', 'barry');
INSERT INTO men VALUES('building2', 'calvin');
INSERT INTO men VALUES(null, 'dwain');
INSERT INTO women VALUES('building1', 'alice');
INSERT INTO women VALUES('building1', 'betty');
INSERT INTO women VALUES(null, 'casandra');

select 
   r1.building_name, 
   r1.men,
   GROUP_CONCAT(women.name) as women,
   COUNT(women.name) +  r1.men_count as count
from 
   (select 
      building.name as building_name,
      GROUP_CONCAT(men.name) as men,
      COUNT(men.name) as men_count 
   from 
      building 
   left join 
      men on building.name=men.building
   GROUP BY building.name) as r1
left join 
   women on r1.building_name=women.building 
GROUP BY r1.building_name;

可能还有另一种方式吗？上述方法的问题是子查询中的两个表的列是隐藏的，需要在外部查询中重新声明。在两个单独的集合操作中执行此操作会创建不对称的情况。我们同样可以先与女性一起，然后是男性。

Answer 1

在SQL Server中，我只会加入两个带有两个左连接的子查询 - 如果你正在寻找对称性：

SELECT *
FROM building
LEFT JOIN (SELECT building, etc. FROM men GROUP BY etc.) AS men_summary
    ON building.name = men_summary.building_name
LEFT JOIN (SELECT building, etc. FROM women GROUP BY etc.) AS women_summary
    ON building.name = women_summary.building_name

我倾向于使用首先声明的公共表表达式而不是子查询 - 它更具可读性（但不是ANSI - 但后来都不是GROUP_CONCAT）。

Answer 2

使用Union来合并男/女表中的数据

select building, [name] as menname, null as womenname from men
union
select building, null as menname, [name] as womenname from women

现在，您可以在子查询中添加一个“表”，您可以加入，计数或其他任何内容。

顺便说一下，我可以看出为什么Cas [s] andra在寒冷中出局，因为没有人相信她，但是dwain呢，他同样被众神诅咒了吗？

是否可以在不使用子查询的情况下报告2个表？

2 个答案: