SQL Server, speaking about ALIAS ...learn the rule

Hello friends!

Welcome back to this short post.
We will speak about Alias and only two minute to learn some important things is really a big deal!

So think big and think that Share knowledge is the only way!

Alias

I think that everyone know what an alias is. We can have a table and call it with another name or we can have a field of a table and call it with another another name.

 
SELECT CODE FROM ORDTES
SELECT CODE as CODICE FROM ORDTES as ORDINI 

Where 'as' is optional
 
Using alias if your statement represents a derived table then you can refer to it through the alias:

 
SELECT * FROM ORTDES 
JOIN 
 (SELECT CODE AS CODICE FROM ORDTES) AS ORDINI ON ORDINI.CODICE = ORDTES.CODE
 
Tips: You can see the Alias also int the execution plan

 

And then..


Suppose now to play with our usual ORDTES table.

Suppose again you need to write a statement to get the max number of document.
Using a not efficient way (i know) we could write:

 
SELECT id FROM ORDTES dad 
WHERE NOT EXISTS (SELECT son.id FROM ORDTES son WHERE son.NUMDOC > dad.NUMDOC)


Obiouvsly we could't write this query with out the Aliases
 
Without alias we can't write a query where the same table name is used in the main query and inside a subquery.

Keep attention we can execute the query below and it run 

     
    SELECT id FROM ORDTES
 WHERE NOT EXISTS (SELECT id FROM ORDTES WHERE NUMDOC > NUMDOC)

But the statemement inside the NOT EXISTS refer only to itself! 

There will never be a row of ordtes where numdoc> numdoc and so the not exists will always return true.


A rule to remember!


Remember the rule:

for each table, SQL Server tries to find a match within the same scope and, if necessary, switches to an external scope




That's all for today! and...  Think big! Share Knowledge!
Luca













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