What's the Best Way to Join on the Same Table Twice

What's the best way to join on the same table twice?

First, I would try and refactor these tables to get away from using phone numbers as natural keys. I am not a fan of natural keys and this is a great example why. Natural keys, especially things like phone numbers, can change and frequently so. Updating your database when that change happens will be a HUGE, error-prone headache. *

Method 1 as you describe it is your best bet though. It looks a bit terse due to the naming scheme and the short aliases but... aliasing is your friend when it comes to joining the same table multiple times or using subqueries etc.

I would just clean things up a bit:

SELECT t.PhoneNumber1, t.PhoneNumber2, 
   t1.SomeOtherFieldForPhone1, t2.someOtherFieldForPhone2
FROM Table1 t
JOIN Table2 t1 ON t1.PhoneNumber = t.PhoneNumber1
JOIN Table2 t2 ON t2.PhoneNumber = t.PhoneNumber2

What i did:

• No need to specify INNER - it's implied by the fact that you don't specify LEFT or RIGHT
• Don't n-suffix your primary lookup table
• N-Suffix the table aliases that you will use multiple times to make it obvious

*One way DBAs avoid the headaches of updating natural keys is to not specify primary keys and foreign key constraints which further compounds the issues with poor db design. I've actually seen this more often than not.

How to join same table twice on Access

In MS Access, more than one JOIN requires parentheses pairings:

SELECT d.departmentID, d.depName, d.location, c1.memberID, 
       c1.fullName,  c1.reportsTo, c2.fullName
FROM (Contacts as c1
INNER JOIN Departments as d
   ON c1.departmentID = d.departmentID)
INNER JOIN Contacts as c2
   ON c1.reprtsTo = c2.memberID

joining the same table twice on different columns

SELECT 
     complaint.complaint_text, 
     A.username, 
     B.username
FROM 
     complaint 
     LEFT JOIN user A ON A.user_id=complaint.opened_by 
     LEFT JOIN user B ON B.user_id=complaint.closed_by

MYSQL: Left join same table twice with same field

this is an example:

SELECT toD.dom_url AS ToURL, 
    fromD.dom_url AS FromUrl, 
    rvw.*

FROM reviews AS rvw

LEFT JOIN domain AS toD 
    ON toD.Dom_ID = rvw.rev_dom_for

LEFT JOIN domain AS fromD 
    ON fromD.Dom_ID = rvw.rev_dom_from

you can use Alias

Inner Join to Same Table Twice on same column

Sorry, I cannot comment. But I believe Peter is right in his comment. Since you are using 2 inner joins they both need to return results. Are you expecting both joins to find a match?

Try this and see which column is null. That is the join that is resulting in no returned rows.

SELECT 
    FAC.Vendedor
    ,REP.RepIDTabacal
    ,REP2.RepIDCtayOrden
FROM 
    ViewFacturacionDiaria_Test AS FAC
LEFT JOIN
    ViewInformacionRepresentantes AS REP ON
    REP.RepIDTabacal = FAC.Vendedor
LEFT JOIN
    ViewInformacionRepresentantes AS REP2 ON
    REP2.RepIDCtayOrden = FAC.Vendedor
WHERE
    FecCpbte BETWEEN '2015-11-28' AND '2015-11-30'

Joining the same table twice in one query

A typical method uses left join and coalesce():

select a.*,
       coalesce(b.col1, c.col1) as col1
from a left join
     b
     on b.ext_id = a._id left join
     c
     on c.ext_id = a._id;

Use the coalesce() for all columns that you want from the two tables.

MySQL: How do I join same table multiple times?

You need to use multiple LEFT JOINs:

SELECT 
    ticket.ticket_id,  
    a1.attr_val AS attr_val1,
    a2.attr_val AS attr_val2,
    a3.attr_val AS attr_val3
FROM ticket
    LEFT JOIN attr a1 ON ticket.ticket_id=a1.ticket_id AND a1.attr_type=1
    LEFT JOIN attr a2 ON ticket.ticket_id=a2.ticket_id AND a2.attr_type=2
    LEFT JOIN attr a3 ON ticket.ticket_id=a3.ticket_id AND a3.attr_type=3

Here is an example: SQL Fiddle.

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