How to declare a variable in mysql, so that my second query can use it?
I would like to write something like:
SET start = 1;
SET finish = 10;
SELECT * FROM places WHERE place BETWEEN start AND finish;
There are mainly three types of variables in MySQL:
User-defined variables (prefixed with
You can access any user-defined variable without declaring it or
initializing it. If you refer to a variable that has not been
initialized, it has a value of
NULL and a type of string.
You can initialize a variable using
SET @start = 1, @finish = 10;
SELECT @start := 1, @finish := 10; SELECT * FROM places WHERE place BETWEEN @start AND @finish;
User variables can be assigned a value from a limited set of data types: integer, decimal, floating-point, binary or nonbinary string, or NULL value.
User-defined variables are session-specific. That is, a user variable defined by one client cannot be seen or used by other clients.
They can be used in
SELECT queries using Advanced MySQL user variable techniques.
Local Variables (no prefix) :
Local variables needs to be declared using
They can be used as local variables and the input parameters inside a stored procedure:
DELIMITER // CREATE PROCEDURE sp_test(var1 INT) BEGIN DECLARE start INT unsigned DEFAULT 1; DECLARE finish INT unsigned DEFAULT 10; SELECT var1, start, finish; SELECT * FROM places WHERE place BETWEEN start AND finish; END; // DELIMITER ; CALL sp_test(5);
DEFAULT clause is missing, the initial value is
The scope of a local variable is the
BEGIN ... END block within
which it is declared.
Server System Variables (prefixed with
The MySQL server maintains many system variables configured to a default value.
They can be of type
Global variables affect the overall operation of the server whereas session variables affect its operation for individual client connections.
To see the current values used by a running server, use the
SHOW VARIABLES statement or
SHOW VARIABLES LIKE '%wait_timeout%'; SELECT @@sort_buffer_size;
They can be set at server startup using options on the command line or in an option file.
Most of them can be changed dynamically while the server is running using
SET GLOBAL or
-- Syntax to Set value to a Global variable: SET GLOBAL sort_buffer_size=1000000; SET @@global.sort_buffer_size=1000000; -- Syntax to Set value to a Session variable: SET sort_buffer_size=1000000; SET SESSION sort_buffer_size=1000000; SET @@sort_buffer_size=1000000; SET @@local.sort_buffer_size=10000;