MySQL DBA Training Session 13. User Variables and Prepared StatementsRAM N SANGWAN

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Agenda

• Introduction

• User Variable Syntax

• Uses of Variables

• User Variable Properties

• Prepared Statements

• Using Prepared Statements

• Preparing a Statement - Example

• Prepare Statement – Considerations

• Executing a Prepared Statement

• Deallocating Prepared Statements

Introduction

• MySQL allows you to assign values to variables and refer to them later.

• This is useful when you want to save the results of calculations for use in subsequentstatements.

• User variables are written as @var_name and may be set to an integer, real, string, or NULLvalue.

User Variable Syntax

• In a SET statement, you can assign a value to a variable using either = or := as theassignment operator:

mysql> SET @var1 ='USA';

mysql> SET @var2 :=‘IND';

• In other contexts, such as in a SELECT statement, use the := assignment operator:

mysql> SELECT @var3 :='CAN';

• A SET statement can perform multiple variable assignments, separated by commas:

mysql> SET @var1 ='USA', @var2 ='GBR', @var3 ='CAN';

• If you refer to an uninitialized variable that has not been assigned a value explicitly, its value isNULL:

mysql> SELECT @var1, @var2, @var3, @var4;

Uses of Variables

• User variables can be used where expressions are allowed.

• However, they cannot be used where a literal value is required.

• For example LIMIT, which requires literal integer arguments, and the filename in LOAD DATA INFILE,which must be a literal string.

• User variables are specifically required when using EXECUTE to execute a preparedstatement.

• Each data value given as a parameter to EXECUTE must be passed as a user variable.

• User variables also are used in LOAD DATA INFILE to hold data values read from a file thatare to be transformed before being loaded into a table.

• User variables are not the same as local variables that you declare in stored routines.

User Variable Properties

• User variable names are not case sensitive. (MySQL 5 onwards.)

• User variables are specific to the client connection within which they are usedand exist only for the duration of that connection.

• A user variable used within a connection cannot be accessed by otherconnections.

• A user variable that has been assigned a non-binary string value has thesame character set and collation as the string.

Prepared Statements

• MySQL Server supports prepared statements, which are useful when youwant to run several queries that differ only in very small details.

• For example, you can prepare a statement, and then execute it multiple times,each time with different data values.

• Besides offering a convenience, prepared statements also provide enhancedperformance because the complete statement is parsed only once by theserver.

• When the parse is complete, the server and client may make use of a newprotocol that requires fewer data conversions than when sending eachstatement individually.

Using Prepared Statements

• In most circumstances, statements are prepared and executed using the programminginterface that you normally use for writing applications that use MySQL.

• However, to aid in testing and debugging, it is possible to define and use prepared statementsfrom within the mysql command-line client.

Using Prepared Statements Contd..

mysql> PREPARE my_stmtFROM ->'SELECT COUNT(*)

FROM CountryLanguage WHERE CountryCode = ?';

mysql> SET @code ='ESP';

EXECUTE my_stmt USING @code;

mysql> SET @code ='RUS';

EXECUTE my_stmt USING @code;

mysql> DEALLOCATE PREPARE my_stmt;

Preparing a Statement

• PREPARE takes two arguments:

◦ a name to assign to the statement once it has been prepared, and

◦ the text of an SQL statement.

• Prepared statement names are not case sensitive.

• The text of the statement can be given either as a literal string or as a user variable containingthe statement.

• The statement may not be complete, because data values that are unknown at preparationtime are represented by question mark ('?') characters that serve as parameter markers.

• At the time the statement is executed, you provide specific data values, one for eachparameter in the statement.

Prepare Statement –Example

• The following example prepares a statement named namepop.

mysql> PREPARE namepop

FROM 'SELECT Name, Population

FROM Country

WHERE Code = ? ';

• If the server finds a problem as it parses the statement during a PREPARE, it returns an errorand does not prepare the statement:

mysql> PREPARE error

FROM ‘SELECT NonExistingColumn

FROM Country

WHERE Code = ? ';

ERROR 1054 (42S22): Unknown column'NonExistingColumn' in'field list'

Prepare Statement – Considerations

• If you PREPARE a statement using a statement name that already exists, the server firstdiscards the prepared statement currently associated with the name, and then prepares thenew statement.

• If the new statement contains an error and cannot be prepared, the result is that no statementwith the given name will exist.

• MySQL does not allow every type of SQL statement to be prepared.

• Those that may be prepared are limited to the following:

o SELECT statements

o Statements that modify data: INSERT, REPLACE, UPDATE, and DELETE

o CREATE TABLE statements

o SET, DO, and many SHOW statements

Executing a Prepared Statement

• If the statement contains any '?' parameter markers, a data value must be supplied for each of

them by means of user variables.

• To execute a prepared statement, initialize any user variables needed to provide parameter

values, and then issue an EXECUTE USING statement:

mysql> SET @var1 ='USA';

mysql> EXECUTE namepop USING @var1;

mysql> SET @var2 ='GBR';

mysql> EXECUTE namepop USING @var2;

mysql> SELECT @var3 :='CAN';

mysql> EXECUTE namepop USING @var3;

• If you refer to a user variable that has not been initialized, its value is NULL :

mysql> EXECUTE namepop USING @var4;

Deallocating Prepared Statements

• Prepared statements are dropped automatically when they are redefined or when you closethe connection.

• However, should you wish to do so, use the DEALLOCATE PREPARE statement:

mysql> DEALLOCATE PREPARE namepop;

• MySQL also provides DROP PREPARE as an alias for DEALLOCATE PREPARE.

Thank You