当一个表为空时，查询在内部连接期间不返回任何内容

Question

I am creating a library management system.

I am trying to query a SQL database through VB.NET with an inner join function, and return data to a datagridview, what I am looking for is a return of something, I do not know the best way to proceed.

I have a table called Onloan that holds basic information about the loan, an id for the user UNID , a ISBN for the book, start and end dates along with returned date, then inner joined with the enrolment table, for the user, and catalogue table, for the books, to create a full picture of what loan is active, for what and by who, but I have a reset function on the program that allows the administrator or senior librarian reset a specific table for example the users table, but the current code will not show any results is the users table is empty, obviously it will not because of the statement that I used, meaning that the dashboard will not be correct

SELECT OnLoan.UNID, OnLoan.ISBN, Catalogue.Title
  , Enrolment.Firstname + ' ' + Enrolment.Lastname As 'Full Name'
  , Enrolment.House + Enrolment.TutorGroup as Form
  , OnLoan.StartDate, OnLoan.EndDate
FROM (Catalogue
INNER JOIN OnLoan ON Onloan.ISBN = Catalogue.ISBN)
INNER JOIN Enrolment ON Enrolment.UNID = OnLoan.UNID
WHERE OnLoan.OnLoan = 1
and OnLoan.EndDate >= CONVERT(datetime, '" & DateTime.Now.ToString("yyyy-MM-dd") & "' )

The image is for the dashboard in the onloan and overdue datagrids I want information even if the data is not completed. Just place a "Unknown user" there or something.

I just want information and references for areas I should look at, if you are able to edit my code and let me use it, that would be great, but any information and help you can provide would be appreciated.

Answer 1

You need to investigate OUTER JOINs. These types of JOINS will return all the results of a JOIN from a specific side regardless if there is a matching item in the other table. I created your tables in postgres and gave it a run.

Table Creation

    CREATE TABLE Catalogue (
        Title VARCHAR,
        ISBN VARCHAR,
        PRIMARY KEY (ISBN)
    );

    CREATE TABLE Enrolment (
        UNID serial PRIMARY KEY,    
        Firstname VARCHAR,
        Lastname VARCHAR,
        House VARCHAR,
        TutorGroup VARCHAR
    );

    CREATE TABLE OnLoan (
        UNID serial PRIMARY KEY,
        ISBN VARCHAR,
        OnLoan integer, 
        StartDate TIMESTAMP DEFAULT NOW(),
        EndDate TIMESTAMP DEFAULT (NOW() + interval '21 days')
    );

Seed the tables with data

    INSERT INTO Catalogue (Title, ISBN) VALUES ('Harry Potter and the Sorcerer''s Stone', '978-0439708180');
    INSERT INTO Catalogue (Title, ISBN) VALUES ('Harry Potter and the Chamber of Secrets', '978-0439064873');
    INSERT INTO Catalogue (Title, ISBN) VALUES ('Harry Potter And The Prisoner Of Azkaban', '978-0439136365');
    INSERT INTO Catalogue (Title, ISBN) VALUES ('Harry Potter And The Goblet Of Fire', '978-0439139601');

    INSERT INTO Enrolment (Firstname, Lastname, House, TutorGroup) VALUES ('Harry','Potter','Gryffindor','Granger Study Group');
    INSERT INTO Enrolment (Firstname, Lastname, House, TutorGroup) VALUES ('Hermione','Granger','Gryffindor','Granger Study Group');
    INSERT INTO Enrolment (Firstname, Lastname, House, TutorGroup) VALUES ('Ronald','Weasley','Gryffindor','Granger Study Group');
    INSERT INTO Enrolment (Firstname, Lastname, House, TutorGroup) VALUES ('Draco','Malfoy','Slytherin','Snape''s Dungeon Group');


    INSERT INTO OnLoan (UNID, ISBN, OnLoan) 
    VALUES (
        (SELECT e.UNID FROM Enrolment e WHERE e.Lastname='Malfoy'),
        (SELECT c.ISBN FROM Catalogue c WHERE c.TITLE='Harry Potter and the Chamber of Secrets'),
        1
    );

Execute the queries and see how adding an outer join can help

You can see that Malfoy has a loan

SELECT \
 o.UNID, o.ISBN, \
 c.Title, \
 CONCAT(e.Firstname, ' ', e.Lastname) full_name, \
 CONCAT(e.House, ' ', e.TutorGroup) form, \
 o.StartDate, o.EndDate \
FROM OnLoan o
 INNER JOIN Enrolment e ON o.UNID=e.UNID
 INNER JOIN Catalogue c ON o.ISBN=c.ISBN
WHERE o.OnLoan = 1 
 AND o.EndDate >= NOW()
;
 unid |      isbn      |                  title                  |  full_name   |              form              |         startdate          |          endda
te           
------+----------------+-----------------------------------------+--------------+--------------------------------+----------------------------+---------------
-------------
    4 | 978-0439064873 | Harry Potter and the Chamber of Secrets | Draco Malfoy | Slytherin Snape' Dungeon Group | 2020-05-09 22:31:36.086555 | 2020-05-30 22:
31:36.086555
(1 row)

Delete Malfoy from the System

DELETE FROM Enrolment WHERE Lastname='Malfoy';
DELETE 1

Run the query again and you cannot seen any loans from Malfoy. He made a clean getaway

SELECT \
 o.UNID, o.ISBN, \
 c.Title, \
 CONCAT(e.Firstname, ' ', e.Lastname) full_name, \
 CONCAT(e.House, ' ', e.TutorGroup) form, \
 o.StartDate, o.EndDate \
FROM OnLoan o
 INNER JOIN Enrolment e ON o.UNID=e.UNID
 INNER JOIN Catalogue c ON o.ISBN=c.ISBN
WHERE o.OnLoan = 1 
 AND o.EndDate >= NOW()
;
unid | isbn | title | full_name | form | startdate | enddate 
------+------+-------+-----------+------+-----------+---------
(0 rows)

What you need to investigate is outer joins. Inner joins means that a record has to exist in both the left and right hand side of the join to appear. With the OUTER JOIN you can see that there is a Loan, but the user is gone.

SELECT \
 o.UNID, o.ISBN, \
 c.Title, \
 CONCAT(e.Firstname, ' ', e.Lastname) full_name, \
 CONCAT(e.House, ' ', e.TutorGroup) form, \
 o.StartDate, o.EndDate \
FROM OnLoan o
 LEFT OUTER JOIN Enrolment e ON o.UNID=e.UNID
 INNER JOIN Catalogue c ON o.ISBN=c.ISBN
WHERE o.OnLoan = 1 
 AND o.EndDate >= NOW()
;
unid |      isbn      |                  title                  | full_name | form |         startdate          |          enddate           
    ------+----------------+-----------------------------------------+-----------+------+----------------------------+----------------------------
4 | 978-0439064873 | Harry Potter and the Chamber of Secrets |           |      | 2020-05-09 22:31:36.086555 | 2020-05-30 22:31:36.086555
(1 row)

Answer 2

If you want to allow relations that have no match in Enrolment , you can use a left join when bringint that table, and coalesce() to assign default values:

SELECT 
    ol.UNID, 
    ol.ISBN, 
    ca.Title, 
    COALESCE(en.Firstname + ' ' + en.Lastname, 'Uknown User') As FullName, 
    COALESCE(en.House + en.TutorGroup, 'Unknown Form') as Form, 
    ol.StartDate, 
    ol.EndDate 
FROM Catalogue ca
INNER JOIN OnLoan ol ON ol.ISBN = ca.ISBN
LEFT JOIN Enrolment en ON en.UNID = ol.UNID 
WHERE 
    ol.OnLoan = 1 
    and ol.EndDate >= cast(getdate() as date)

Side notes:

• do not use single quotes for column aliases - they are meant for string literals - instead, use square brackets (or better yet, use column aliases that do not require quoting)

• nested parentheses around join s are superfluous in SQL Server (only MS Access requires that)

• table aliases make the query easier to read and write

• no need to use a parameter to get the current date, you can do that directly in the query

Answer 3

I did more research and it was full outer join that I wanted, I just kept overlooking it as I kept looking at it wrong.