Every programming language enables some method for declaring local (or global) variables that can be used to store data...

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Smutek to uczucie, jak gdyby się tonęło, jak gdyby grzebano cię w ziemi.

Transact-SQL is no exception. Declaring a
variable using Transact-SQL is an extremely simple procedure. The keyword that must be
used is the DECLARE keyword. The syntax looks like this:
SYNTAX:
declare @variable_name data_type
To declare a character string variable to store players' names, use the following
statement:
1> declare @name char(30)
2> go
Note the @ symbol before the variable's name. This symbol is required and is used by the query processor to identify variables.
Declaring Global Variables
If you delve further into the Transact-SQL documentation, you will notice that the @@
symbol precedes the names of some system-level variables. This syntax denotes SQL
Server global variables that store information.
Declaring your own global variables is particularly useful when using stored
procedures. SQL Server also maintains several system global variables that contain
information that might be useful to the database system user. Table 19.4 contains the
complete list of these variables. The source for this list is the Sybase SQL Server System 10 documentation.
Table 19.4. SQL Server global variables.
Variable Name
Purpose
@@char_convert
0 if character set conversion is in effect.
@@client_csid
Client's character set ID.
@@client_csname
Client's character set name.
@@connections
Number of logons since SQL Server was started.
Amount of time, in ticks, the CPU has been busy since SQL Server
@@cpu_busy
was started.
@@error
Contains error status.
@@identity
Last value inserted into an identity column.
Amount of time, in ticks, that SQL Server has been idle since
@@idle
started.
@@io_busy
Amount of time, in ticks, that SQL Server has spent doing I/O.
@@isolation
Current isolation level of the Transact-SQL program.
@@langid
Defines local language ID.
@@language
Defines the name of the local language.
@@maxcharlen
Maximum length of a character.
Maximum number of connections that can be made with SQL
@@max_connections
Server.
@@ncharsize
Average length of a national character.
@@nestlevel
Nesting level of current execution.
@@pack_received
Number of input packets read by SQL Server since it was started.
Number of output packets sent by SQL Server since it was
@@pack_sent
started.
Number of errors that have occurred since SQL Server was
@@packet_errors
started.
@@procid
ID of the currently executing stored procedure.
@@rowcount
Number of rows affected by the last command.
@@servername
Name of the local SQL Server.
@@spid
Process ID number of the current process.
@@sqlstatus
Contains status information.
Maximum length of text or image data returned with SELECT
@@textsize
statement.
@@thresh_hysteresis Change in free space required to activate a threshold.
@@timeticks
Number of microseconds per tick.
@@total_errors
Number of errors that have occurred while reading or writing.
@@total_read
Number of disk reads since SQL Server was started.
@@total_write
Number of disk writes since SQL Server was started.
@@tranchained
Current transaction mode of the Transact-SQL program.
@@trancount
Nesting level of transactions.
@@transtate
Current state of a transaction after a statement executes.
@@version
Date of the current version of SQL Server.
Using Variables
The DECLARE keyword enables you to declare several variables with a single statement (although this device can sometimes look confusing when you look at your code later).
An example of this type of statement appears here:
1> declare @batter_name char(30), @team int, @average float
2> go
The next section explains how to use variables it to perform useful programming
operations.
Using Variables to Store Data
Variables are available only within the current statement block. To execute a block of
statements using the Transact-SQL language, the go statement is executed. (Oracle uses the semicolon for the same purpose.) The scope of a variable refers to the usage of the
variable within the current Transact-SQL statement.
You cannot initialize variables simply by using the = sign. Try the following statement and note that an error will be returned.