Question

我有2个表，表名是用户和项目。表的结构是：

用户表

id | name   | role  
1  | samjad | user  
2  | saneer | constructor

项目表

id | name   | user_id | constructor_id |  
1  | school | 1       | 2              |

如何根据项目表ID从一个表中获取所有详细信息。

我想选择

projectname username, constroctorname, user_id, constroctor_id

在一行

Answer 1

事实上你在项目表和用户之间有两个关系（一个用户用户，一个用于constroctor）你可以使用用户连接两次

  select p.name, u1.username, u2.username, p.user_id, p.constroctor_id
  from projects as p
  inner join user as u1 on p.user_id = u1.id
  inner join user as u2 on p.constroctor_id = u2.id

Answer 2

您可以将用户表加入两次 - 一次作为用户，然后作为构造函数。

select p.name as projectname,
    u.name as username,
    c.name as contructorname,
    p.user_id,
    p.contructor_id
from projects p
left join user u on p.user_id = u.id
left join user c on p.contructor_id = c.id
where u.role = 'user'           -- check if the said user has role "user"
    and c.role = 'constructor'; -- check if the said constructor has role "constructor"

Answer 3

您可以使用concat（）函数：

SELECT CONCAT(field1, field2, field3);

https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_concat

或CONCAT_WS（分隔符，str1，str2，...）

Answer 4

假设有一个名为Constructor的表，其中包含列名和constructor_id，那么您的查询将是

select
  p.name as projectname,
  u.name as username,
  c.name as constructorname,
  u.id as userid,
  c.id as constructorid
from 
projects p 
inner join user u on p.user_id=u.id
inner join constructor c on p.constructor_id=c.id

Answer 5

使用联接。你可能想要一个INNER JOIN：

SELECT * -- actually include only the fields you need
FROM Projects p
INNER JOIN Users u ON u.id = p.user_id
INNER JOIN Users uc ON uc.id = p.constructor_id

Answer 6

您需要在其键上加入表格。在这种情况下，如下所示：

select p.name as projectname
     , u.name as username
     , if(u.role='constructor',u.name,null) as constructorname
     , p.user_id, p.constructor_id
  from users u
  join projects p
    on p.user_id = u.id;

mysql连接两行合为一

6 个答案: