Comments on: Nested LEFT JOINs to link three or more tables

By: Ryan

Ryan — Tue, 14 Oct 2008 21:29:08 +0000

Yes, that’s where I did this query.

By: Rufor

Rufor — Sun, 12 Oct 2008 16:02:38 +0000

can i use it in MySQL queries?

By: Jonathan Snook

Jonathan Snook — Mon, 22 Aug 2005 19:33:38 +0000

You shouldn’t have had to nest them to do what you wanted. The following should do the same:

SELECT `accounts`.`name` AS account_name, `contacts`.`name` AS contact_name, `date`, `amount`, `type`, `detail`, `memo`, `cleared` FROM `transactions` LEFT JOIN `accounts` ON `accounts`.`id` = `account_id` LEFT JOIN `contacts` ON `contacts`.`id` = `contact_id` WHERE `parent_id` = 0

By: Bernie Zimmermann

Bernie Zimmermann — Mon, 22 Aug 2005 16:25:28 +0000

It’s funny how these things just seem to pop up at the same time…

Database Joins

By: Bernie Zimmermann

Bernie Zimmermann — Mon, 22 Aug 2005 04:14:37 +0000

I understand now. I was letting the actual data being queried get in the way of the problem. Thanks for clarifying.

By: Ryan

Ryan — Sun, 21 Aug 2005 14:58:56 +0000

Bernie, I think you misunderstand the information I am going after. Here is the situation: I have a long list of transactions, which may or may not have associated contact people and account information. I want the transactions to display even if the contact person is not defined, and even if the account is not defined. In other words, I want to display all of the transactions, no matter what. But if aditional information happensto be associated with a transaction, I’d like that information as well. In a two-table case, this is a normal straightforward LEFT JOIN. The result is that the table on the left returns all of its rows, even if there is no matching entry in the table on the right. Let me give you an example of where this is neccesary:

Lets say you have a database of students at a school. You have all of the student names in one table, ’students’. Then you have a table called ‘absences.’ In that table, you store a date and a student_id - this information can be combined to return how many absences each student has:

SELECT name, absences FROM students, absences WHERE student_id = students.id GROUP BY students.id;

This would return a list of all students that have absences. But what if I wanted a list of all students including those that have no absences? Then I use a LEFT JOIN:

SELECT name, absences FROM students LEFT JOIN absences ON student_id = students.id GROUP BY students.id;

This nifty query returns a row for every student, even if there are no matching records in the absences table. Clearly, LEFT JOIN serves a useful purpose.

However, I wanted to take it a step further. In my table, I have transactions that may or may not have an associated account, and that may or may not have an associated contact. I want all the transactions, even if the account is not defined, and even if the contact is not defined. But in the case that the account or the contact is defined, I want that information too. So the nested LEFT JOIN makes this possible.

I hope that clears things up.

By: Bernie Zimmermann

Bernie Zimmermann — Sat, 20 Aug 2005 01:49:18 +0000

I’m unclear on why you would be interested in the transaction information along with the accounts and contacts information but not care if they match up. The only reason I can think of to include the accounts and contacts information is if it matches up with the rows you’re fetching from transactions.

If you don’t care about accounts and contacts information, simply leave those two tables out of the query.

There’s gotta be an easier way to do what you’re doing, but I can’t quite figure out the reasoning behind what you’re doing in the first place ;)

I’ve had to query some pretty crazy data before and have never had to do the kind of thing you’re demonstrating here.