Question

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Explicit vs implicit SQL joins

I have two queries in SQL Server.

First query:

Select * 
From Stack, Overflow
Where Stack.Id = Overflow.StackId

Second query:

Select * 
From Stack 
Inner Join Overflow On Overflow.StackId = Stack.Id

These two queries return the same results.

So what is the difference between this two queries in terms of performance?

And which one do you prefer?

Answer 1

Use query #2 – it’s the proper ANSI/ISO SQL Standard JOIN syntax, and is preferred over #1.

For one: your JOIN condition is where it belongs – on the JOIN – and doesn’t clutter up your WHERE clause. Your WHERE clause should contain only things that you actually use to constrain your result set.

And secondly: since you have to define your JOIN condition on the JOIN, you’re less likely to “forget” about it and unwantingly produce a cartesian product.

And lastly: since you define INNER JOIN or LEFT OUTER JOIN, your query becomes more readable and easier to udnerstand – for someone else looking at your query, and for yourself in six months when you have to go back and maintain your code.