mysql 两行合二为一

Question

我有 2 tables table name is users 和 projects.the table 的结构是：

用户table

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

项目table

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

如何根据项目 table id.

在单行中从两个 table 获取所有详细信息

我想select

projectname username, constroctorname, user_id, constroctor_id

一行

Answer 1

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 2

假设有一个名为 Constructor 的 table，其列名和 constructor_id 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 3

事实上你在项目table和用户之间有两个关系（一个给用户，一个给constroctor）你可以使用用户加入两次

  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 4

使用联接。您可能需要一个 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 5

您可以加入用户 table 两次 - 一次作为用户，然后作为构造者。

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 6

您将希望通过键加入表格。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 两行合二为一

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