从sql db中的多个表中提取数据

Question

Hi i have a database which consists of 2 tables

userData table

userID  |  userPass | Firstname |  Surname | Status | IPAddress
--------------------------------------------------------------  
110110  |  123456   | James     |  Jackson | Online |  11.112.1

contacts table

    userID  |  contactID |
    --------------------- 
    110110  |  112114    |
    112112  |  110110    |
    110110  |  112444    |

The first table is the userdata table which I am using with my java application for login. I also have another table which stores the users contacts , I am trying to write a query to allow me to take the users contacts and display them by the first name, surname and ID in a JList, I am extremely puzzled any help would be appreciated. I had a look at inner joins but seem to be getting more stuck. how can I construct a query to get the data i need? I am also trying to make a separate query to display only the online users.

ok i have managed to use the following query:

SELECT userName
FROM userdata, contacts
WHERE userID = "110110" AND userData.userID = contacts.contactID

This is taking the userID "110110" from the contacts tables and the contactID's which correspond and returning the userName (from the userdata table)of the contacts. i have encountered a new problem where userID 112112 added 110110 as a friend. because of this 112112 is in the userID column and 110110 is in the contactID column (contacts table). this shouldnt be a problem as it still shows they are contacts. however I have tried to modify my query to also give me the name for the userID 112112. can someone please help me with this?

Answer 1

name, surname and ID

Assuming by ID you mean contactID, otherwise additional join should be included to fetch data from Contacts.

select u.Firstname, u.Surname, uc.contactID from User u 
inner join UserContact uc on u.userID = uc.userID;

More on joins

Answer 2

To get all the users -

select Firstname, Surname, u.contactID from userdata u inner join usercontacts uc 
on u.userID = uc.userID order by Firstname, Surname, u.userID 

To get online users -

select Firstname, Surname, u.contactID from userdata u inner join usercontacts uc 
on u.userID = uc.userID where status = 'Online' order by Firstname, Surname, u.userID