SQL - Check if data exists on either table

问题:

I have two tables in my database regarding the login details for both of my users (Librarians and Students) I have separated the user's details into 2 separate tables tblUserLibrarian and tblUserStudent; one for the Librarians and one for the Students. The Primary key in each table is the username or ID (LibrarianID and StudentID).

Outline of the tables are like this:

tblUserStudent

StudentID   Password     FirstName    LastName

S1201235    pass3           Jane       Smith
S1201289    pass5           Pass       Word
S1202009    pass2           John       Smith

tblUserLibrarian

LibrarianID Password

L1094565    pass4
L1202836    password123
L1202908    qwerty
L1212345    pass3

I have a problem where when either user tries to login using their username (which is their ID). I want to check if their username exists on either table and return all the fields from the table on which the username exists. Currently I can only think of one way which is to do 2 queries to check this however I feel that this can be done in one query. I've looked up JOINS and UNIONS but everything I've tried doesn't seem to work.

My most recent query was:

SELECT TOP 1 * FROM tblUserStudent,tblUserLibrarian
WHERE StudentID = "S1202836" OR LibrarianID = "S1202836"

But this returns rows from both tables, I just want to return the details of the user from one table if the username they entered exists.

I am using MS Access 2010 to do my queries.

回答1:

Your tables don't have the same structure. You could do a UNION ALL to do the query on both tables, but only return some information for Librarians:

SELECT TOP 1 * 
FROM(
  SELECT studentId AS userID, password, firstName, LastName
  FROM tblUserStudent 
  WHERE StudentID = 'S1201235'
  UNION ALL
  SELECT LibrarianID,password, NULL, NULL
  FROM tblUserLibrarian 
  WHERE LibrarianID = 'S1201235'
) a

sqlfiddle demo (sql server, but serves as an example)

I added an alias to the id's column to show you userID instead of studentID, since UNION takes the column names from the first SELECT.

I also left the TOP 1, but if your ID's are unique, you should receive only one, making it irrelevant

回答2:

select StudentID,Password,FirstName,LastName from tblUserStudent where studentID='S1202836'
union
select LibrarianID,Password,null,null from tblUserLibrarian where LibrarianID='L1202836'

I'm not sure if i understand your requirement correctly, but it seems you want a union, not a join, and since your librarian table has fewer columns than your user student table, you have to fill up with null columns so the column count matches.