Comments on: Back to basics: self joins

By: Sukaina

Sukaina — Thu, 31 May 2007 07:36:48 +0000

As stated by Eddie: WHERE d1.department_id IN (10, 20, 30, 40) AND d2.department_id IN (10, 20, 30, 40); What if the department_ids are around 100 in number, would it not be a roblem to enter all department_ids in the query. Using Oracle I can solve it, however using ANSI do we have to use a subquery there as in Oracle or is there a seperate way. Please let me know.

By: Eddie Awad

Eddie Awad — Wed, 12 Jul 2006 16:46:39 +0000

Yas, you're right, the corrected statement that describes

WHERE e.manager_id = m.employee_id (+)

is:

The (+) operator following m.employee_id means that you want to display a row from the e table, even though there exists no corresponding row in the m table.

Thanks for catching it.

By: Nelson

Nelson — Wed, 12 Jul 2006 14:27:38 +0000

Just a bit off topic. I clicked on the Spain’s flag on top of the screen and the text it showed make no sense at all! The damn thing even translate the code so it is a bit futile to run that translation.

By: yas

yas — Wed, 12 Jul 2006 09:12:49 +0000

The (+) operator following m.employee_id means that you want to display a row from the m table, even though there exists no corresponding row in the e table

Eddie, i think it must be the other way around. This code gets all rows from e, not m. This (+) is usually confusing to many people, it makes the sql get all the rows from the table on the other side.

By: Eddie Awad

Eddie Awad — Wed, 12 Jul 2006 01:45:10 +0000

Hi Rob,

The difference between using JOIN/ON and just using WHERE is that the first uses the ANSI/ISO SQL92 standard syntax, and the second uses the good old Oracle syntax.

I refer you to this post for some discussion about the difference between the two and the advantages/disadvantages of using the ANSI SQL syntax (make sure you read the comments).

By the way,

FROM employees e LEFT OUTER JOIN employees m
   ON e.manager_id = m.employee_id

translates to

    FROM employees e, employees m
 WHERE e.manager_id = m.employee_id (+)

The (+) operator following m.employee_id means that you want to display a row from the m table, even though there exists no corresponding row in the e table, and that's what the LEFT OUTER JOIN above means.

By: Robert Vollman

Robert Vollman — Tue, 11 Jul 2006 22:25:25 +0000

What is the difference between using JOIN/ON and just using WHERE?

For example, take your second query, and instead make it FROM employee e, employee m WHERE e.manager_id(+) = m.employee_id

I've never used the JOIN/ON syntax. Does it have an advantage?