Q
Problem solve Get help with specific problems with your technologies, process and projects.

# LEFT OUTER JOIN with ON condition or WHERE condition?

## I would like your explanation of the difference between a LEFT OUTER JOIN ON condition and a WHERE condition.

I would like your explanation of the difference between a LEFT OUTER JOIN ON condition and a WHERE condition. Thank...

you.

The difference is subtle, but it is a big difference. The ON condition stipulates which rows will be returned in the join, while the WHERE condition acts as a filter on the rows that actually were returned.

Simple example: Consider a student table, consisting of one row per student, with student id and student name. In a second table, a list of grades that students have received, with student_id, subject, and grade. Give me a list of all students, and show their grade in Math. This requires a LEFT OUTER JOIN, because you want all students, and you know that some of them didn't take Math. Here are two queries:

```select name
from students
left outer
on students.id
```select name
from students
left outer
on students.id

Now for the crucial difference: the first query returns only those students who took Math, and those who didn't are not included. In the second query, all students are included, and those who took Math have their grade shown.

Why the difference? In the first query, the LEFT OUTER JOIN returns all students, even if they didn't take Math. If they didn't take Math, then the joined row that is returned by the LEFT OUTER JOIN will have NULLs in all the columns from the grades table. But then for each such joined row returned, the WHERE clause comes along and picks only those rows which are Math. And since NULL isn't equal to anything, students who didn't take Math disappear from the results.

In the second query, the join condition means that students who took Math are returned, or else NULL because it's a LEFT OUTER JOIN. So all students are included in the results, because there's no WHERE clause to filter them out. Their Math grade will be their Math grade or else NULL.

In effect, the first query behaves the same as an inner join. Only the matched rows are retained after the WHERE clause has done its job. Why bother returning rows to the WHERE clause that you want filtered out? Make it an INNER JOIN and save some needless processing. Of course, if you do want a LEFT OUTER JOIN, make sure that any filter conditions on the right table are in the ON clause, not the WHERE clause.

This was last published in September 2005

## Content

Find more PRO+ content and other member only offers, here.

#### Have a question for an expert?

Get answers from a TechTarget expert on whatever's puzzling you.

You will be able to add details on the next page.

#### Join the conversation

Send me notifications when other members comment.
Great explanation, thanks sir!
Cancel
im giving code like this:

left outer JOIN ekko
on lfa1~lifnr = ekko~lifnr
where ekko~bukrs = comp_code AND ekko~ekorg = purch_org

and its giving me error:

no fields from the right-hand table of a left-outer join may appear in the WHERE condition : "EKKO~BUKRS"
Cancel

## SearchDataManagement

• ### Hyperledger Fabric offers path to enterprise blockchain future

Blockchain arose from bitcoin, but it's looking to find a place in the enterprise. Frameworks like Hyperledger Fabric could ...

• ### MongoDB 4.0 takes ACID transactions to multi-document level

MongoDB is taking a deeper step into SQL-style processing waters with a 4.0 update that brings increased support for ...

• ### Data lake concept needs firm hand to pay big data dividends

Data lakes pose technology deployment and data management challenges that can leave analytics users high and dry if the ...

• ### AI functionality limited today but could be a game-changer

Limited AI capabilities could soon give way to technology that is truly transformative for enterprises, surpassing the overhyped ...

## SearchSAP

• ### SAP's Barry Padgett on future of SAP Ariba Network

In this Q&A, new SAP Ariba President Barry Padgett discusses the future of procurement and the experience he will bring to Ariba ...

• ### Avoiding SAP indirect access woes requires good faith

Some customers are concerned that SAP will hit them for indirect access licensing fees, but they can avoid trouble if they act in...

• ### ControlPanelGRC app eases Steelcase's compliance pain

When Steelcase's SAP environment grew in size and complexity, it turned to Symmetry ControlPanelGRC to save time, have more ...

## SearchSQLServer

SQL Operations Studio simplifies routine administration of SQL Server and Azure SQL databases, making database development and ...

• ### Meltdown and Spectre fixes eyed for SQL Server performance issues

Microsoft has responded to the Spectre and Meltdown chip vulnerabilities with patches and other fixes. But IT teams need to sort ...

• ### Five SQL Server maintenance steps you should take -- ASAP

Putting off SQL Server administration tasks can lead to database problems. Enact these often-neglected maintenance items to help ...

## TheServerSide.com

• ### How DevOps concepts eluted from cloud computing and service platforms

The popularity of DevOps can be traced back to the emergence of cloud computing. As programmers began scripting their ...

• ### Pluralsight IQ, Stack Overflow boost developer street cred

Tying the Pluralsight IQ skills test to the Stack Overflow Developer Story helps developers measure their technical skills and ...

• ### Why this quantum computing breakthrough is a security risk

Quantum computing will void pretty much all security encryption techniques and open the door to hackers. Here's how to protect ...

## SearchDataCenter

• ### IBM Power9 servers seek market inroads to AI, cloud

IBM follows up its first Power9 server with a raft of systems designed to appeal to a wider array of markets -- most notably, AI ...

• ### Evaluate read-intensive and write-intensive SSD use cases

Consider write wear, performance and other factors when choosing between read-intensive, write-intensive and mixed-use ...

• ### Some hyper-converged infrastructure use cases pose pitfalls

Hyper-converged infrastructure adoption is skyrocketing, but that doesn't mean that the technology is the best choice for every ...

## SearchContentManagement

• ### Content management in the cloud a main theme in 2018

The future of content management resides in the cloud and with AI, as several 2018 conferences will assure you.

• ### Six things to know about today's SharePoint implementations

As companies migrate their on-premises Microsoft SharePoint sites to the cloud, here are some things they should know about the ...

• ### Upgrades for the SharePoint Online portal

As more organizations migrate SharePoint sites to the cloud, Microsoft has increased at-a-glance dashboard data and analytics to ...

## SearchHRSoftware

• ### HR plays to win with gamification for learning

At some companies, corporate education has become a tool for engagement and employee experience. See how gamification is putting ...

• ### HR recruiting tools get VC funding, as tech wages rise

The HR recruiting tools market shows no shortage of investor interest. In two months, three platforms have launched. Rising tech ...

• ### Social media key to future of learning and development

Employees aren't just wasting time on social media; some are learning the career skills that will take them to the next level. ...

Close