Solved

SQL inner vs left - precedence

Posted on 2013-12-20
1
883 Views
Last Modified: 2013-12-20
Hi,
 In MS SQL 2008, I am looking at a sample code like below:

Select *
from table1  t1 inner join table2 t2 on t1.id = t2.id

inner join table3 t3 on t1.id = t3.id

left join table4 t4 on t1.id = t4.id


I wanted to understand which join takes precedence:
1. Do all the three inner join execute first and then the resultset is left joined to the last table? In this case, the precedence is by virtue of which join is written first?

2. Inner joins take precedence over left join , regardless of in which order they are submitted?

3. Left join takes precedence over the inner join?

thx
0
Comment
Question by:LuckyLucks
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
1 Comment
 
LVL 10

Accepted Solution

by:
PadawanDBA earned 500 total points
ID: 39732593
Join order evaluates left to right.  The result of the previous join is the left input to the next join.  Where this can introduce unintended consquences is when you have an outer join before an inner join and then you join to the table that was outer joined.  If there are null values that are preserved from the outer join and those values are used to join another table via an inner join, they will be filtered out.  You can get around this by parenthetically grouping your joins together to force the inner join to be evaluated before the outer join.
0

Featured Post

U.S. Department of Agriculture and Acronis Access

With the new era of mobile computing, smartphones and tablets, wireless communications and cloud services, the USDA sought to take advantage of a mobilized workforce and the blurring lines between personal and corporate computing resources.

Question has a verified solution.

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

This post looks at MongoDB and MySQL, and covers high-level MongoDB strengths, weaknesses, features, and uses from the perspective of an SQL user.
It is possible to export the data of a SQL Table in SSMS and generate INSERT statements. It's neatly tucked away in the generate scripts option of a database.
This is my first video review of Microsoft Bookings, I will be doing a part two with a bit more information, but wanted to get this out to you folks.
In this video you will find out how to export Office 365 mailboxes using the built in eDiscovery tool. Bear in mind that although this method might be useful in some cases, using PST files as Office 365 backup is troublesome in a long run (more on t…

630 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