Hi,

I came across a query at SQL Alias I can't understand. It says:

SELECT po.OrderID, p.LastName, p.FirstName
FROM Persons AS p,
Product_Orders AS po
WHERE p.LastName='Hansen' AND p.FirstName='Ola'

From what I see, there's no "JOIN" keyword used, but it's apparently going to operate on two separate tables. If it's going two retrieve data from "Product_Orders" tables, why isn't it mentioned after "WHERE" at all? So, how could "p.LastName=" and "p.FirstName" be related to "po.OrderID", how it knows what rows should be picked from "Product_Orders" table?

[SIGH] yet another example of crappy sql from w3schools...

QUESTION: how it knows what rows should be picked from "Product_Orders" table?

ANSWER: by the absence of any filtering or join criteria, it picks all of them




does this make sense? no, but then, this example was about table aliases only

Thanks, so the result will be like this, right?

OrderID|LastName|FirstName
1|Hansen|Ola
2|Hansen|Ola
3|Hansen|Ola
4|Hansen|Ola
5|Hansen|Ola
...

To add to Rudy's comments, as there is no join between the Persons and Product_Orders table you will have "Cartesian Join". This is where for every row in one table will join with every row from a second table resulting in large amounts of data. In some cases this is desirable, in other cases not.

As for no JOIN the above statement could be rewritten as follows:

right


cartesian joins aren't necessarily bad if there are WHERE clause conditions

most optimizers are smart enough to push those conditions up into the data retrieval stage



did that make sense to anyone but me?

Ok thank you for answers