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Question

我有2张桌子

表 1：户

Serial_no	Address
sn1	New York
sn2	Maryland
sn3	France

表 2：市民

Serial_id	Fullname	Role	household_id
1	John	Head	sn1
2	Jane	Spouse	sn1
3	Johny	Son	sn1
4	Mike	Head	sn2

我希望输出是这样的：

Serial_no	Address	Total_count	Head
sn1	New York	3	John
sn2	Maryland	1	Mike
sn3	France	0	null

我被困在这里了。请帮忙。提前致谢！

Answer 1

您可以使用 group by 的条件聚合，如下所示：

架构和插入语句：

 create table households(Serial_no varchar(10),    Address varchar(50));

 insert into households values('sn1',            'New York');
 insert into households values('sn2',            'Maryland');
 insert into households values('sn3',            'France');

 create table citizens(Serial_id int, Fullname varchar(50), Role varchar(10), household_id varchar(10));

 insert into citizens values(1,'John', 'Head',    'sn1');
 insert into citizens values(2,'Jane', 'Spouse', 'sn1');
 insert into citizens values(3,'Johny', 'Son',       'sn1');
 insert into citizens values(4,'Mike', 'Head',     'sn2');

查询：

 select Serial_no,Address,count(c.household_id) Total_count,
 max(case when Role='Head' then Fullname end) Head
 from households h
 left join citizens c
           on h.Serial_no=c.household_id
 group by Serial_no,Address

输出：

Serial_no	Address	Total_count	Head
sn1	New York	3	John
sn2	Maryland	1	Mike
sn3	France	0	null

db<>fiddle here

左加入额外信息

LEFT JOIN with EXTRA INFORMATION

sql

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