MySQL: Grouping results with JOIN

Posted on 2016-09-21
Last Modified: 2016-09-25!9/437c3/1/0

I need these results to be returned:
10 Smiles | 179736 | 1
Apple Hill Stories | 86019 | 0
Finding Phil | 179736 |1
Hand and Foot | 179736 | 1
Raising the Roof | (null) |(null)
Silly Sally | 86019 | 0

Open in new window

However, "10 Smiles" is included in the results 3 times.  Each item should only be included once, and if there is a match for ContactID = 179736 then X should be "1"
Question by:skij
LVL 52

Assisted Solution

by:Julian Hansen
Julian Hansen earned 50 total points
ID: 41808657
Which contactID should be used if there is more than one match and there is no 179736 in the results.
LVL 10

Author Comment

ID: 41808726
If there is no match for 179736 then the contactID should be null, as it is for "Raising the Roof"
LVL 22

Expert Comment

by:Ferruccio Accalai
ID: 41808761
So how do you explain these result?
Apple Hill Stories | 86019 | 0
Silly Sally | 86019 | 0

There's something missing in your explanation or maybe we're misunderstanding the concept.
LVL 10

Author Comment

ID: 41808842
Thank you for asking for clarification.  I want a list of Names for all available books, and I want to know if customer 179736  has already purchased that book or not.

This example should better demonstrate my need.

I have attached a sample dump.

SELECT `ZBooks`.`Name`,
    WHEN `ZOrders`.`ContactID` = 179736  THEN 'Customer 179736 Purchased This Book'
    else '-'
    END AS `Status`
from `ZBooks` 
Left Join `ZOrders` on `ZOrders`.`CourseID` = `ZBooks`.`CourseID`
ORDER BY `ZBooks`.`Name` ASC

Open in new window

The problem with this query is that one of the books is included three times in the list but it should only be included once.  

I want results like this:
10 Smiles		|	Customer 179736 Purchased This Book
Apple Hill Stories	|	-
Finding Phil		|	Customer 179736 Purchased This Book
Hand and Foot	|	Customer 179736 Purchased This Book
Raising the Roof	|	-
Silly Sally		|	-

Open in new window

PRTG Network Monitor: Intuitive Network Monitoring

Network Monitoring is essential to ensure that computer systems and network devices are running. Use PRTG to monitor LANs, servers, websites, applications and devices, bandwidth, virtual environments, remote systems, IoT, and many more. PRTG is easy to set up & use.

LVL 22

Expert Comment

by:Ferruccio Accalai
ID: 41808865
I was working on a solution during your post so maybe what I'm suggesting is not exactly what you need but it's really near to your goal.

This is my query to get the result needed in first sight:
SELECT ZBooks.Name, ZOrders.ContactID, ContactID =179736 AS X
LEFT JOIN ZOrders ON ZBooks.CourseID = ZOrders.CourseID
) Z1
SELECT ZBooks.Name, ZOrders.ContactID, ContactID =179736 AS X
LEFT JOIN ZOrders ON ZBooks.CourseID = ZOrders.CourseID
ORDER BY ZBooks.Name ASC) Z2 ON ( Z1.Name = Z2.Name AND Z1.X < Z2.X ) 

Open in new window

LVL 10

Author Comment

ID: 41808903
Thank you, Ferruccio Accalai.  That produces the result set that I am looking for.  It uses 3 SELECT statements and 3 JOIN statements.   Is that the most performance efficient way to get these results?
LVL 22

Accepted Solution

Ferruccio Accalai earned 350 total points
ID: 41808927
In your case that's the only way to get the desired result. As You need to get just 1 record per book, what you have to do is a join between the first given query and another query where you skip the unwanted records. That's the normal way to solve a query matter like yours
LVL 40

Assisted Solution

Sharath earned 100 total points
ID: 41809769
try this.
SELECT b.Name, o.ContactID, o.ContactID = 179736 as X
  FROM ZBooks b
                    IF(@CourseID = CourseID, @rn := @rn+1, @rn := 1) rn,
                    IF(@CourseID <> CourseID, @CourseID := CourseID, -9999) dummy
               FROM ZOrders,(SELECT @rn := 1, @CourseID := -9999) t1
              ORDER BY CourseID, CASE ContactID WHEN 179736 THEN 0 ELSE 1 END, ContactID) o
   ON b.CourseID = o.CourseID AND o.rn = 1;

Open in new window!9/437c3/17

Featured Post

Complete VMware vSphere® ESX(i) & Hyper-V Backup

Capture your entire system, including the host, with patented disk imaging integrated with VMware VADP / Microsoft VSS and RCT. RTOs is as low as 15 seconds with Acronis Active Restore™. You can enjoy unlimited P2V/V2V migrations from any source (even from a different hypervisor)

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

A lot of articles have been written on splitting mysqldump and grabbing the required tables. A long while back, when Shlomi ( had suggested a “sed” way, I actually shell …
This guide whil teach how to setup live replication (database mirroring) on 2 servers for backup or other purposes. In our example situation we have this network schema (see atachment). We need to replicate EVERY executed SQL query on server 1 to…
Windows 10 is mostly good. However the one thing that annoys me is how many clicks you have to do to dial a VPN connection. You have to go to settings from the start menu, (2 clicks), Network and Internet (1 click), Click VPN (another click) then fi…
This is used to tweak the memory usage for your computer, it is used for servers more so than workstations but just be careful editing registry settings as it may cause irreversible results. I hold no responsibility for anything you do to the regist…

920 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

16 Experts available now in Live!

Get 1:1 Help Now