Conditional JOIN different tables

If you look at employee and student tables as one, you can use left join:

select * 
from user u
left join 
(
   select 'Employee' as UserType,
          id,
          user_id
     from employee e 
    union all
   select 'Student',
          id,
          user_id
     from student s 
) r
  ON u.user_id = r.user_id

If you want to get all user data together You might have:

SELECT 
    user_id
    ,'Employee' AS Source
FROM 
    employee
UNION 
SELECT 
    user_id
    ,'Student' AS Source
FROM 
    student

http://sqlfiddle.com/#!3/90216/22

Which can also be done with a full join and a CASE statement:

SELECT 
    ISNULL(e.user_id,s.user_id) AS user_id
    ,CASE WHEN e.user_id IS NULL THEN 'Student' 
        ELSE 'Employee'
    END AS SOURCE
FROM 
    employee AS e 
    FULL JOIN student AS s
        ON s.user_id = e.user_id

http://sqlfiddle.com/#!3/90216/29

the latter will combine people who are both students adn employees into one row and call them and employee. compare:

http://sqlfiddle.com/#!3/2aa3e/1 and http://sqlfiddle.com/#!3/2aa3e/2

where I have made user 1 a student and a employee

You could use an outer join:

select *
  from USER u
  left outer join EMPLOYEE e ON u.user_id = e.user_id
  left outer join STUDENT s ON u.user_id = s.user_id
 where s.user_id is not null or e.user_id is not null

alternatively (if you're not interested in the data from the EMPLOYEE or STUDENT table)

select *
  from USER u
 where exists (select 1 from EMPLOYEE e where e.user_id = u.user_id)
    or exists (select 1 from STUDENT s  where s.user_id = u.user_id)

Such solution also can help you.

SELECT  S.*, P.*

,CASE
    WHEN S.ShipmentType = 'import' THEN SP.SupplierName
    WHEN S.ShipmentType = 'export' THEN C.CustomerName
END AS ShipmentDesination

FROM            tblShippments   S 
INNER JOIN      tblProducts     P   ON S.productId = P.productID  
LEFT OUTER JOIN tblCustomers    C   ON S.companyId = C.customerId AND S.ShipmentType = 'export'
LEFT OUTER JOIN tblSuppliers    SP  ON S.companyId = SP.supplierId AND S.ShipmentType = 'import'