Question

在我的mysql数据库中，我有两个表，project和sub_project。子项目包含id和首字母缩略词;子项目有一个project_id，它告诉它属于哪个主项目。数据库如下所示：

s/\(jdbc-uri *= *\).*/\1"jdbc:mysql:\/\/xxxx"/g
s/\(jdbc-password *= *\).*/\1"xxxx"/g
s/\(jdbc-username *= *\).*/\1"xxxx"/g

我想创建一个查询，显示每个项目的id和首字母缩略词，以及子项目的数量，如下所示：

mysql> SELECT id, acronym FROM project;
+----+---------+
| id | acronym |
+----+---------+
| 14 | P1      |
| 15 | P2      |
| 16 | P3      |
+----+---------+
3 rows in set (0.00 sec)

mysql> SELECT id, project_id FROM subproject;
+----+------------+
| id | project_id |
+----+------------+
|  1 |         14 |
|  2 |         14 |
+----+------------+
2 rows in set (0.00 sec)

但是，这在语法上是不正确的。该如何制定？

Answer 1

架构：

create table project
(   id int not null,
    acronym varchar(100) not null
);
insert project values
(14,'P1'),
(15,'P2'),
(16,'P3');

create table subproject
(   id int not null,
    project_id int not null
);
insert subproject values
(1,14),
(2,14);

具有子项目的项目：

select p.id,p.acronym,count(s.id) as theCount 
from project p 
join subproject s 
on s.project_id=p.id 
group by p.id,p.acronym; 

+----+---------+----------+
| id | acronym | theCount |
+----+---------+----------+
| 14 | P1      |        2 |
+----+---------+----------+

所有项目甚至没有子项目的项目：

select p.id,p.acronym,count(s.id) as theCount 
from project p 
left join subproject s 
on s.project_id=p.id 
group by p.id,p.acronym; 

+----+---------+----------+
| id | acronym | theCount |
+----+---------+----------+
| 14 | P1      |        2 |
| 15 | P2      |        0 |
| 16 | P3      |        0 |
+----+---------+----------+

用subselect替换select中的列名

1 个答案: