MySql连接问题3表

Question

I am trying to join a 3 tables. projects, statuses, clients. The GOAL of the query should result in all projects that have a current status of "Received". I am also trying to grab the client name for displaying in the resulting table.

These tables have common foreign keys as: client_ID, status_ID.

Here is my query thus far. I am trying hard to get a better understanding of joins. If you can provide comments on what I am doing wrong and example code that would be very much appriciated.

   SELECT clients.clientName, 
          clients.Client_ID, 
          projects.Client_ID,
          projects.projectNumber,
          projects.projectName, 
          projects.expectedDate,
          statuses.Status_ID, 
          statuses.status         
     FROM projects, 
          clients, 
          statuses 
LEFT JOIN clients on projects.Client_ID = clients.Client_ID 
LEFT JOIN statuses on projects.Status_ID = statuses.Status_ID 
    WHERE status = 'Received'

Answer 1

Try using 'JOIN' instead of 'LEFT JOIN'. Also remove clients and statuses in the from clause. I would also query the Status_ID instead of the status name.

SELECT clients.clientName, 
          clients.Client_ID, 
          projects.Client_ID,
          projects.projectNumber,
          projects.projectName, 
          projects.expectedDate,
          statuses.Status_ID, 
          statuses.status         
     FROM projects
     JOIN clients on projects.Client_ID = clients.Client_ID 
     JOIN statuses on projects.Status_ID = statuses.Status_ID 
     WHERE statuses.status = 'Received'

Answer 2

just try with below code.

Remove LEFT JOIN and just use WHERE condition with INNER JOIN...

SELECT clients.clientName, 
          clients.Client_ID, 
          projects.Client_ID,
          projects.projectNumber,
          projects.projectName, 
          projects.expectedDate,
          statuses.Status_ID, 
          statuses.status         
     FROM projects, 
          clients, 
          statuses 
      WHERE 
          clients on projects.Client_ID = clients.Client_ID AND
          statuses on projects.Status_ID = statuses.Status_ID AND
          status = 'Received'

This may be helpful to you and give records what you are suppose to waiting for.

Answer 3

You should only have one table-name in the FROM-clause, or else you'll be joining tables multiple times (ANSI-syntax). Also it helps to set the join-conditions the other way around (easier to read I think); and to expand the table names (projects.status=...) instead of (status=...).

Also, as already pointed out, "JOIN" instead of "LEFT JOIN" would force corresponding rows to exist if the 'projects' row are to be returned.

SELECT clients.clientName, clients.Client_ID, projects.Client_ID,
       projects.projectNumber,projects.projectName, projects.expectedDate,
       statuses.Status_ID, statuses.status 

FROM projects

LEFT JOIN clients ON clients.Client_ID = projects.Client_ID 

LEFT JOIN statuses ON statuses.Status_ID = projects.Status_ID

WHERE statuses.status = 'Received'