Name
DAT-06: Employ “user” variables for global data sparingly
Synopsis
A global variable is a data structure
that can be referenced outside the scope or block in which it’s
declared. In MySQL, we can use “user” variables—which can be
recognized by being prefixed with @
—to set values that are available to
any program within the current session.
In the following procedure, for example, we store the number
of customers into the user variable @customer_count
:
CREATE PROCEDURE sp_customer_count( )
SELECT COUNT(*)
INTO @customer_count
FROM customers;
Other procedures can examine the @customer_count
and make decisions
without having to recalculate the value. For instance, in this
procedure we use the session variable in our setup logic:
CREATE PROCEDURE sp_crm_setup ( ) BEGIN IF @customer_count IS NULL THEN CALL sp_customer_count( ); END IF; IF @customer_count > 1000 THEN . . . Logic for larger enterprises . . ..
There is no doubt that the use of global variables can create easy solutions for difficult problems. However, the modern consensus is that global variables create their own problems and that these problems generally overwhelm any of the advantages they might confer.
Global variables defeat modularity and hinder code reuse, because any module that uses a global variable becomes dependent on some other module that creates or initializes the global variable. In the case of MySQL user variables—which don’t require a formal declaration—there is also the chance that two programmers might ...
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