for example if I declared one variable like - Declare #Test Int, where the SQL server store this variable?
SQL Variables are not stored anywhere on the server. They are session variables that are omitted once the session is closed. You can only use a SQL variable in the same session you declared it, within the block in which it is declared.
Related
I don't know how to operate MySQL DECLARE THIS TABLE?
MS_SQL CAN
DECLARE #TEMP TABLE
(variable INT)
But MySQL can't do this! How can i do it?
Syntax error !!
MySQL does not have the feature of declaring a variable for a table. Variables can only be scalars.
If you need a feature that Microsoft SQL Server supports, then you should use Microsoft SQL Server.
Also when you use DECLARE in MySQL, you can't use the # sigil on variables. Local variables in stored routines don't have that sigil in MySQL. This is another difference from Microsoft SQL Server.
If you need to make a table or database name come from an argument or variable, you will need to
Construct the query, probably using CONCAT.
PREPARE the query.
EXECUTE the query.
DEALLOCATE PREPARE.
Suppose I defined two variables in mysql as follows:
SET #STUDENT_NAME = 'ABCD';
SET #STUDENT_AGE = 25;
Now, When I'm was selecting these two variables value using following:
SELECT #STUDENT_NAME, #STUDENT_AGE;
It was perfectly showing corresponding values, even if I switched the database.
But when I restarted mysql and tried to select these values, It was showing null values of these variables, So where would it be stored actually when I had set values?
Per the manual (emphasis mine):
https://dev.mysql.com/doc/refman/8.0/en/user-variables.html
User-defined variables are session specific. A user variable defined by one client cannot be seen or used by other clients. (Exception: A user with access to the Performance Schema user_variables_by_thread table can see all user variables for all sessions.) All variables for a given client session are automatically freed when that client exits.
MySQL allows usage of User-defined variables How to declare a variable in MySQL?
Is there a way for me to pre-set one such constant, so that it's automatically ready to use in PHP scripts, CLI clients and whatnot? As if every session were automatically started with a
SET #whatever = 2.52;
Create a table, that keeps this value and select from it everywhere, when you need it. That will be a simple subselect in sql queries.
MySQL 5.6
CREATE PROCEDURE test()
BEGIN
DECLARE _idKeep INT;
SET _idKeep = 1;
PREPARE string FROM "UPDATE users set firstname='Joe' where id=?";
EXECUTE string USING _idKeep;
/*SELECT _idKeep;*/
END
Error Message:
You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use
near '_idKeep;
If I comment out the PREPARE and EXECUTE statement, and uncomment the SELECT statement, then the error message goes away. Why would that be?
You cannot use local variable in your query, user defined variable only.
Parameter values can be supplied only by user variables, and the USING clause must name exactly as many variables as the number of parameter markers in the statement.
http://dev.mysql.com/doc/refman/5.7/en/execute.html
User-defined variables are session-specific. A user variable defined by one client cannot be seen or used by other clients. (Exception: A user with access to the Performance Schema user_variables_by_thread table can see all user variables for all sessions.) All variables for a given client session are automatically freed when that client exits.
http://dev.mysql.com/doc/refman/5.7/en/user-variables.html
Because local variables are in scope only during stored program execution, references to them are not permitted in prepared statements created within a stored program. Prepared statement scope is the current session, not the stored program, so the statement could be executed after the program ends, at which point the variables would no longer be in scope. For example, SELECT ... INTO local_var cannot be used as a prepared statement. This restriction also applies to stored procedure and function parameters.
http://dev.mysql.com/doc/refman/5.7/en/local-variable-scope.html
A prepared statement created within a stored program continues to exist after the program finishes executing and can be executed outside the program later.
A statement prepared in stored program context cannot refer to stored procedure or function parameters or local variables because they go out of scope when the program ends and would be unavailable were the statement to be executed later outside the program. As a workaround, refer instead to user-defined variables, which also have session scope.
http://dev.mysql.com/doc/refman/5.7/en/prepare.html
You could use user-defined variable and restrict edition of it anywhere but this function.
I wanted to know what is the difference between #variablename and variablename. All the help would be appreciated.
variablename
Local variables
Local variables are declared within stored procedures and are only valid within the BEGIN…END block where they are declared. Local variables can have any SQL data type.
#variablename
User variables
In MySQL stored procedures, user variables are referenced with an ampersand (#) prefixed to the user variable name (for example, #x and #y). You can store a value in a user-defined variable in one statement and then refer to it later in another statement. This enables you to pass values from one statement to another. User-defined variables are session-specific. That is, a user variable defined by one client cannot be seen or used by other clients. All variables for a given client session are automatically freed when that client exits.
In MySQL the variable var1 and #var1 are two different things. #var1 is a user session variable which remains for the duration of the active session. The other is local to the stored procedure.