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  #1  
Old 04-05-2009
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Join Date: May 2008
Posts: 923
The concept of SQL Join

I just spent two hours of tutorials on joins, including the right to develop, try to understand what is a join and what it does. I think I am far from everything, and I write this post to try to get ideas. What I would like to ask you, is to correct me in these statements that I present below:

a join is a relationship of equality between one or several bases.
a natural join is equivalent to time, a performance and logical of a "WHERE table1.value = table2.value".

An outer join is equivalent to a natural join with the exception that it includes all entries in the table specified after FROM for a left outer join and right outer join after a right outer join. It is used to verify equal while in the selection encompassing all entries compared with the field is NULL.

I confess that I have been very difficult to grasp the concept, and more than I have no way (or right) to test any code, or see the consequences. If you have comments to make, try, please be clear.
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  #2  
Old 04-05-2009
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Join Date: Feb 2008
Posts: 194
Re: The concept of SQL Join

Quote:
a join is a relationship of equality between one or several bases.
No, I do not know what you mean but this is not "between several bases".

Quote:
a natural join is equivalent to time, a performance and logical of a "WHERE table1.value = table2.value".
Basically yes
It is also equivalent to a
Code:
INNER JOIN Table2 
ON table1.value = table2.value
Quote:
An outer join is equivalent to a natural join with the exception that it includes all entries in the table specified after FROM for a left outer join and right outer join after a right outer join.
No, an outer join is equivalent to an inner join. You can have a

Code:
FROM Table1  
LEFT OUTER JOIN Table2 
ON Table1.Field1 = Table2.Field2
Quote:
It is used to verify equal while in the selection encompassing all entries compared with the field is NULL.
Not compared with the field has no match in the other table.
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  #3  
Old 04-05-2009
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Join Date: Feb 2008
Posts: 1,848
Re: The concept of SQL Join

Forget for the time natural join, it is ease of writing that does nothing.

Concentrate first on your inner join, also known as joint strong. Attack then on cross join, the Cartesian product. The concept and syntax are very simple. Once you understand that you can attack the principle of outer join.

You do not sense the right way, we must think in data set, not sure of syntax in writing before or after a word.
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  #4  
Old 04-05-2009
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Join Date: May 2008
Posts: 923
Re: The concept of SQL Join

Quote:
Originally Posted by Ashok.M View Post
No, an outer join is equivalent to an inner join. You can have a

Code:
FROM Table1  
LEFT OUTER JOIN Table2 
ON Table1.Field1 = Table2.Field2
But then what is the difference between an inner join and outer join?

Quote:
Originally Posted by Ashok.M View Post
Not compared with the field has no match in the other table.
But then what is a restricted join? Which fields are filtered? on what criteria?
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