Chapter -02 Declaring PL/SQL Variables - 02

Types of Variables

 

Guidelines for Declaring and Inititalizing PL/SQL Variables

• Follow naming conventions.
• Use meaningful identifiers for variables.
• Initialize variables designated as NOT NULL and CONSTANT.
• Initialize variables with the assignment operator (:=) or the DEFAULT keyword:

v_myName VARCHAR2(20) := 'John';
v_myName VARCHAR2(20) DEFAULT 'John';

• Declare one identifier per line for better readbility and code maintenance.

Guidelines for Declaring PL/SQL Variables

• Avoid using column names as identifiers.

 PL/SQL里面，如果列名和变量名相同，优先PL/SQL引擎优先解析为列名.

DECLARE employee_id　　NUMBER(6);

BEGIN

　　SELECT employee_id--字段名称
   
　　INTO 　 employee_id--变量名称

　　FROM employees

　　WHERE last_name = 'Kochhar';

END;

/

• Use the NOT NULL constrain when the variable must hold a value.

Scalar Data Types

• Hold a single value
• Have no iternal components

Base Scalar Data Types

• CHAR [(maximum_length)]
• VARCHAR2(maximum_length)
• NUMBER [(precision,scale)]
• BINARY_INTEGER
• PLS_INTEGER
• BOOLEAN
• BINARY_FLOAT
• BINARY_DOULBE
• DATE
• TIMESTAMP
• TIMESTAMP WITH TIME ZONE
• TIMESTAMP WITH LOCAL TIME ZONE
• INTERVAL YEAR TO MONTH
• INTERVAL DAY TO SECOND

Declaring Scalar Variables

Examples:

DECLARE
    v_emp_job           VARCHAR2(9);
    v_count_job         BINARY_INTEGER :=0;
    v_dept_total_sal    NUMBER(9,2)  :=0;
    v_orderdate         DATE :=SYSDATE + 7;
    v_tax_rate          CONSTANT NUMBER(3,2) :=8.25;
    v_valid             BOOLEAN NOT NULL :=TRUE;
    ...

%TYPE Attribute

• Is used to decalre a variable according to:
  • -A database column definition
  • -Another declared variable
• Is prefixed with:
  • -The database table and column
  • -The name of the decared variable

Declaring Variables with the %TYPE  Attribute

• Syntax

identifier table.column_name%TYPE;

• Examples

...

　　employee_name employees.last_name%TYPE;

...

...

　　balance　　NUMBER(7,2);

　　min_balance balance%TYPE :=1000;

...

Declaring Boolean Variables

• Only the TRUE,FALSE,and NULL values can be assigned to a Boolean variable.
• Conditional expressions use the logical operators AND and OR and the unary operator NOT to check the variable values.
• The variables alwarys yield TRUE,FALSE,or NULL.
• Airthmetic,character,and date expressions can be used to return a Boolean value.

在PL/SQL中，Boolean型变量，不同于其他语言，其包含三种类型的值，分别是(TRUE,FALSE,NULL)；

IF(TRUE)

....

IF(FALSE | NULL)

...

Bind Variables

Bind variables are:

• Created in the enviroment
• Also called host variables.
• Created with the VARIABLE keyword.
• Used in SQL statements and PL/SQL blocks
• Accessed even after the PL/SQL block is executed
• Referenced with a preceding colon.

声明Bind Variables，使用关键字VARIABLE;绑定变量属于非PL/SQL变量.

PL/SQL声明变量,使用关键字DECLARE.

日常开发中，应当注意区别运用DECLARE 和 VARIABLE关键字，声明变量.通过绑定变量和PL/SQL，实现他们彼此之间的交互.

Pinting Bind Variables

Example:

VARIABLE b_emp_salary NUMBER
BEGIN
        SELECT salary INTO :b_emp_salary
        FROM employees WHERE employee_id = 178;
END;
/

PRINT b_emp_salary;
SELECT first_name,last_name FROM employees
WHERE salary = :b_emp_salary;

SQL> @bind.sql

PL/SQL procedure successfully completed.


B_EMP_SALARY
------------
        7000


FIRST_NAME           LAST_NAME
-------------------- -------------------------
Oliver               Tuvault
Sarath               Sewall
Kimberely            Grant

VARIABLE b_emp_salary
SET AUTOPRINT ON
DECLARE
        v_empno NUMBER(6) :=&empno;
BEGIN
        SELECT salary INTO :b_emp_salary
        FROM employees WHERE employee_id = v_empno;
END;

/
SQL> @bind2.sql
variable   b_emp_salary
datatype   NUMBER
Enter value for empno: 100
old   2:        v_empno NUMBER(6) :=&empno;
new   2:        v_empno NUMBER(6) :=100;

PL/SQL procedure successfully completed.


B_EMP_SALARY
------------
       24000

LOB Data Type Variables

Composite Data Types

Summary

In this lesson,you should have learned how to:

• Recognize valid and invalid identifiers.
• Declare variables in the declarative section of a PL/SQL block.
• Initialize variables and use them in the executable section.
• Differentiate between scalar and composite data types.
• Use the %TYPE attribute
• Use bind variables.