Mysql如何联接2个表和1个表本身

Question

I have a table name tbl_user this table contain

db_uid   db_fname   db_lname   db_responsibleid     db_email
  1         xxx       yyy            0             xxx@x.com
  2         ttt       eee            1             ttt@x.com
  3         ggg       hhh            2             ggg@x.com

The other table is tbl_task this table contain

db_taskid    db_mytask    db_desc   db_status   db_emailed   db_userid
   1           test       test        Done          0           2
   2           bnvc       sdcs        Done          0           3
   3           bcv        sdc         Scheduled     0           3

Now i want to have this result:

  taskid   mytask   desc   Status   staffname   responsiblename    remail
    1       test    test   Done      ttt eee      xxx yyy        xxx@x.com
    2       bnvc    sdcs   Done      ggg hhh      ttt eee        ttt$x.com

this result to select all task that have db_status='Done' and db_emailed='0' with the name of the user and the name of the responsible of that user with the email of the responsible

I use this query but i can't get the name of the responsible and his email address

select 
    tbl_task.db_mytask,
    tbl_task.db_desc,
    tbl_task.db_taskid,
    tbl_task.db_status,
    concat(tbl_user.db_fname,' ' ,tbl_user.db_lname) as name,
    concat(user.db_fname,' ' ,user.db_lname) as uname,
    user.db_email
from tbl_task 
left join tbl_user
    on tbl_task.db_userid=tbl_user.db_uid
left join tbl_user as user
    on user.db_uid=user.db_responsibleid
where 
    (tbl_task.db_emailed='0') 
    and 
    (tbl_task.db_status='Done')

Current result is:

taskid   mytask   desc   Status   staffname   responsiblename    remail
  1       test    test   Done      ttt eee        NULL             NULL
  2       bnvc    sdcs   Done      ggg hhh        NULL             NULL

How can i solve this problem any suggestion or help ?!

Answer 1

You join the wrong columns on the second left join (i have use user1 and use2 alias for the two join on tbl_user)

you use user.db_uid=user.db_responsibleid but should tbl_user.db_uid=user.db_responsibleid

  select 
    tbl_task.db_mytask,
    tbl_task.db_desc,
    tbl_task.db_taskid,
    tbl_task.db_status,
    concat(user1.db_fname,' ' ,user1.db_lname) as name,
    concat(user2.db_fname,' ' ,user2.db_lname) as uname,
    user2.db_email
  from tbl_task 
  left join tbl_user as user1 on tbl_task.db_userid=user1.db_uid
  left join tbl_user as user2 on user2.db_uid=user1.db_responsibleid
  where tbl_task.db_emailed='0'
  and tbl_task.db_status='Done'

Answer 2

You can try this query. Check if it meets your requirements

select
    tbl_task.db_taskid,
    tbl_task.db_mytask as mytask, 
    tbl_task.db_desc,    
    tbl_task.db_status,
    concat(tbl_user.db_fname,' ' ,tbl_user.db_lname) as name,
    concat(users.db_fname,' ' ,users.db_lname) as responsiblename, 
    users.db_email as remail
from tbl_user AS users 
    RIGHT OUTER JOIN tbl_user ON users.db_uid = tbl_user.db_responsibleid 
    RIGHT OUTER JOIN tbl_task ON tbl_user.db_uid = tbl_task.db_userid
where  (tbl_task.db_emailed='0') and (tbl_task.db_status='Done')