What does @ /@@ mean?

Question

I'm trying to find a clear explanation for what @ or @@ mean in T-SQL as I can't figure this out...

Answer 1

@ refers to the local variables you create with DECLARE statement, for example, declare @TotalCount int, @StartDate datetime

The parameters in functions and stored procedures also have @ in front of them.

@@ refers to system defined functions, such as @@ROWCOUNT, @@Error, for example.

See http://msdn.microsoft.com/en-us/library/ms187953.aspx for more info.

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Premature optimization is the root of all evil in programming. (c) by Donald Knuth

Naomi Nosonovsky, Sr. Programmer-Analyst

My blog

Answer 2

Local variable names are prefixed with @. Some T-SQL system function names begin with @@ (sometimes they are called global variables but in fact they are functions).

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Plamen Ratchev

Answer 3

This may be stating the obvious (or not) but do I need to CREATE TABLE with column names before running a DECLARE statement?

I am guessing that a local variable refers to a column name and property within the database. So is global variable (in the old sense) columns outside the current db.?

It's going to take me a little time to get my head around this...

Answer 4

No, the @variable is just like a variable in any other language, say, in C# you would declare new int variable

int myIntVar; //you don't need to initialize it right away

In T-SQL the analogue would be

declare @myIntVar int

In SQL Server 2008 you can declare and initialize in one statement:

declare @myIntVar = 5

Please read the link I gave you earlier to understand how to declare variables in T-SQL and how to declare table variables.

You don't need CREATE table before running a declare - they serve different purposes.

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Premature optimization is the root of all evil in programming. (c) by Donald Knuth

Naomi Nosonovsky, Sr. Programmer-Analyst

My blog

Answer 5

 Hi,

Identifier starting with @ you can use in declaring the variable(local variables)
but   identifier starting with @@ you cant use in declaring the variable because these are global variable (system defined)

Eg:
@@row_count
@@fetch_status

Regards
Vipin Nair

Answer 6

Thanks - this is beginning to make sense now. I also asked someone who uses SQL Server at work & he showed me some DECLARE statements using @ and functions using @@, so this is much clearer now.

Answer 7

you can use it also in creating a table like this.

declare @myTable as table
(
testRow INT identity(1,1)
)

Regards,

Sphinx