Summary: in this tutorial, you will learn about variables in stored procedure, how to declare, and use variables. In addition, you will learn about the scopes of variables.
A variable is a named data object whose value can change during the stored procedure execution. We typically use the variables in stored procedures to hold the immediate results. These variables are local to the stored procedure.
You must declare a variable before you can use it.
To declare a variable inside a stored procedure, you use the
DECLARE statement as follows:
DECLARE variable_name datatype(size) DEFAULT default_value;
Let’s examine the statement above in more detail:
- First, you specify the variable name after the
DECLAREkeyword. The variable name must follow the naming rules of MySQL table column names.
- Second, you specify the data type of the variable and its size. A variable can have any MySQL data types such as
- Third, when you declare a variable, its initial value is
NULL. You can assign the variable a default value by using
For example, we can declare a variable named
total_sale with the data type
INT and default value
0 as follows:
DECLARE total_sale INT DEFAULT 0;
MySQL allows you to declare two or more variables that share the same data type using a single
DECLARE statement as following:
DECLARE x, y INT DEFAULT 0;
We declared two integer variables
y , and set their default values to zero.
Once you declared a variable, you can start using it. To assign a variable another value, you use the
SET statement, for example:
DECLARE total_count INT DEFAULT 0;
SET total_count = 10;
The value of the
total_count variable is
10 after the assignment.
SET statement, you can use
SELECT INTO statement to assign the result of a query, which returns a scalar value, to a variable. See the following example:
DECLARE total_products INT DEFAULT 0
SELECT COUNT(*) INTO total_products
In the example above:
- First, we declare a variable named
total_productsand initialize its value to
- Then, we used the
SELECT INTOstatement to assign the
total_productsvariable the number of products that we selected from the
productstable in the sample database.
A variable has its own scope that defines its life time. If you declare a variable inside a stored procedure, it will be out of scope when the
END statement of stored procedure reached.
If you declare a variable inside
BEGIN END block, it will be out of scope if the
END is reached. You can declare two or more variables with the same name in different scopes because a variable is only effective in its own scope. However, declaring variables with the same name in different scopes is not good programming practice.
A variable that begins with the
@ sign is session variable. It is available and accessible until the session ends.
In this tutorial, we have shown you how to declare a variable inside stored procedures and discussed about the variable scopes.