Tài liệu developing a simple PL / SQL docx

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć2Schedule: Timing Topic 60 minutes Lecture 30 minutes Practice 90 minutes Total... Introduction to Oracle: SQL and PL/SQ

Developing a Simple PL/SQL

Block

21

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć2

Schedule: Timing Topic

60 minutes Lecture

30 minutes Practice

90 minutes Total

In this lesson, you create a simple PL/SQL block after learning the various elements that compose a block.

At the end of this lesson, you should be able to

D Declare and use variables and constants in PL/SQL

D Assign new values to variables within the executable section

D Create and execute a named PL/SQL subprogram in Procedure Builder

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć4

A PL/SQL block is comprised of up to three sections: declarative (optional),

executable (required), and exception handling (optional) Only BEGIN and ENDkeywords are required Each subprogram contains an additional section, the header(required)

You can store and change values within a PL/SQL block by declaring and referencingvariables and other identifiers

Handling Variables

D Declare and initialize variables within the declaration section

D Assign new values to variables within the executable section

D Pass values into PL/SQL blocks through parameters

D View the results from a PL/SQL block through output variables

Note: The END keyword can be optionally followed by the name of the subprogram

for clarity

Class Management Note:

PowerPoint: The bottom slide contains the build feature

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć6

Declaring PL/SQL Variables and Constants

You need to declare all identifiers within the declaration section before referencing

them within the PL/SQL block

Syntax

identifier [CONSTANT] datatype [NOT NULL]

[:= | DEFAULT expr];

where: identifier is the name of the identifier

CONSTANT constrains the identifier so that its value cannot

change; constants must be initialized

datatype is a scalar or composite datatype

NOT NULL constrains the variable so that it must contain a

value; NOT NULL variables must be initialized

expr is any PL/SQL expression that can be a literal,

another variable, or an expression involvingoperators and functions

Guidelines

D Name the identifier according to the same rules used for SQL objects

D You can use naming conventions, for example v_name to represent a variable, and

c_name to represent a constant.

D You have the option of assigning an initial value to variables, unless they areNOT NULL

D Initialize the variable to an expression with the assignment operator (:=), or,equivalently, with the DEFAULT reserved word; otherwise, variables are

initialized to NULL by default.

D Declare at most one identifier per line

For more information, see

PL/SQL User’s Guide and Reference, Release 2.3, “Datatypes.”

Technical Note:

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć8

Class Management Note:

Mention that the NUMBER, CHAR, and VARCHAR2 datatypes havesubtypes Additional base types are RAW, ROWID, and MLSLABEL(Trusted Oracle)

Declaring Scalar Variables

PL/SQL supports three datatypes—scalar, composite, and reference—that you canuse for declaring variables, constants, and pointers

Scalar Datatypes

A scalar datatype holds a single value and has no internal components Scalar

datatypes can be classified into four categories: number, character, date and time, orBoolean Character and number datatypes have subtypes that associate a base type to

a constraint For example, INTEGER and POSITIVE are subtypes of the NUMBERbase type

BINARY_INTEGER Base type for integers between –2147483647

and 2147483647

NUMBER [(precision,scale)] Base type for fixed and floating point numbers

CHAR [(maximum_length)] Base type for fixed length character data up to

32767 bytes If you do not specify a

maximum_length, the default length is set to 1.

LONG Base type for variable length character data up

to 32760 bytes

LONG RAW Base type for binary data up to 32760 bytes

VARCHAR2(maximum_length) Base type for variable length character data up

to 32767 bytes

DATE Base type for dates and times

BOOLEAN Base type that stores one of three possible

values used for logical calculations: TRUE,FALSE, or NULL

Note: The above list is abridged For the complete list, see the PL/SQL User’s Guide

and Reference, Release 2.3, “Datatypes.”

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć10

Class Management Note:

Declaring Scalar Variables continued

Declare a constant for the tax rate, which never changes throughout the PL/SQLblock

Declare a flag to indicate whether a piece of data is valid or invalid, and initialize thevariable to TRUE

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć12

Declaring Scalar Variables continued

The %TYPE Attribute

When you declare PL/SQL variables to hold column values, you must ensure that thevariable is of the correct datatype and precision If it is not, then a PL/SQL error willoccur during execution

Rather than hard-coding the datatype and precision of a variable, you can declare avariable according to another previously-declared variable or database column You

do this using the %TYPE attribute To use the attribute in place of the datatype

required in the variable declaration, prefix it with the database table and columnnames If referring to a previously-declared variable, prefix the variable name to theattribute

PL/SQL determines the datatype and size of the variable when the block is compiled,

so the variable is always compatible with the database column or identifier used topopulate the variable

Advantages of Using the %TYPE Attribute

D The datatype of the underlying database column may be unknown

D The datatype of the underlying database column may change at runtime

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć14

Declaring Composite Datatypes

A composite datatype contains internal components and is reusable Two types of

composite datatypes are available in PL/SQL: TABLE and RECORD

PL/SQL Table

D A PL/SQL TABLE datatype is not the same as a database table

D A PL/SQL TABLE is similar to a one-dimensional array

D A PL/SQL TABLE must contain two components:

D A primary key of datatype BINARY_INTEGER that indexes the PL/SQLTABLE

D A column of a scalar datatype, which stores the PL/SQL TABLE elements

D A PL/SQL TABLE can increase dynamically because it is unconstrained

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć16

Declaring Composite Datatypes continued

Declaring PL/SQL Tables

1. Declare a TABLE datatype

2. Declare a variable of that datatype

Syntax

TYPE type_name IS TABLE OF scalar_datatype [NOT NULL]

INDEX BY BINARY_INTEGER;

where: type_name is the name of the TABLE type

scalar_datatype is the datatype of the PL/SQL TABLE elements

You can use the %TYPE attribute

identifier is the name of the identifier

The NOT NULL constraint prevents nulls from being assigned to the PL/SQL

TABLE of that type Do not initialize the PL/SQL TABLE

Class Management Note:

PowerPoint: The top slide uses the build feature

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć18

Declaring Composite Datatypes continued

PL/SQL RECORD

D A PL/SQL RECORD datatype is not the same as a row in a database table

D A PL/SQL RECORD is similar in structure to a record in a 3GL

D A PL/SQL RECORD must contain one or more components of any scalar,

RECORD, or PL/SQL TABLE datatype called fields These uniquely namedfields can have different datatypes

D The PL/SQL RECORD allows you to treat this collection of fields as one logicalunit

D PL/SQL RECORDS are convenient for fetching a row of data from a table forprocessing in a PL/SQL block

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć20

Declaring Composite Datatypes continued

Declaring PL/SQL Records

1. Declare a RECORD datatype

2. Declare a variable of that datatype

where: type_name is the name of the RECORD type

field_name is the name of the field

field_type is the datatype of the field You can use the

%TYPE and %ROWTYPE attribute

expr is any PL/SQL expression that can be a literal,

another variable, or an expression involvingoperators and functions

identifier is the name of the identifier

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć22

Declaring Composite Datatypes continued

Declaring Records withthe %ROWTYPE Attribute

Declare a record based upon a collection of columns in a database table or view byusing the %ROWTYPE attribute The fields within the record take their names anddatatypes from the columns of the table or view

Advantages of Using the %ROWTYPE Attribute

D The number and datatypes of the underlying database columns may be unknown

D The number and datatypes of the underlying database columns may change atruntime

D Useful when retrieving a row with the SELECT statement

Class Management Note:

Draw on the board the structure of DEPT_RECORD

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć24

PL/SQL Block Syntax Rules

Because PL/SQL is an extension of SQL, the general syntax rules that apply to SQLare also applicable to the PL/SQL language

D Statements can be split across lines, but keywords must not be split

D Lexical units (for example, identifiers or literals) can be separated by one or morespaces or other delimiters that cannot be confused as being part of the lexical unit

D Character and date literals must be enclosed within single quotation marks

D Numeric literals can be represented by either a simple value (for example, –32.5)

or by scientific notation (for example, 2E5, meaning 2x10 to the power of 5 =200000)

D Multiple line comments can be enclosed by /* and */ symbols A single linecomment begins with , and the end-of-line marks the comment’s end

For more information, see

PL/SQL User’s Guide and Reference, Release 2.3, “Appendix E” for a list of reserved

words

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć26

PL/SQL Block Syntax Rules continued

Nested Blocks

One of the advantages PL/SQL has over SQL is the ability to nest statements Youcan nest blocks wherever an executable statement is allowed, thus making the nestedblock a statement Therefore, you can break down the executable part of a block intosmaller blocks The exception section can also contain nested blocks

Variable Scope

The scope of an object is the region of the program that can refer to the object You

can reference the declared variable within the executable section

An identifier is visible in the block in which it is declared and all nested sub-blocks,procedures, and functions If the block does not find the identifier declared locally, it

looks up to the declarative section of the enclosing (or parent) blocks The block never looks down to enclosed (or child) blocks or sideways to sibling blocks.

Scope applies to all declared objects, including variables, cursors, user-defined

exceptions, and constants

Class Management Note:

Qualify an identifier by using the block label prefix This topic is covered inthe lesson “Controlling Flow in PL/SQL Blocks.”

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć28

Assigning Values to Variables

To assign or reassign a value to a variable, you write a PL/SQL assignment statement.You must explicitly name the variable to receive the new value to the left of theassignment operator (:=)

Syntax

identifier := expr;

plsql_table_name(primary_key_value) := expr;

plsql_record_name.field_name := expr;

where: identifier is the name of the identifier

plsql_table_name is the name of the PL/SQL TABLE

primary_key_value is the binary integer value that is the index of

the PL/SQL TABLE and could also be avariable

plsql_record_name is the name of the PL/SQL RECORD

field_name is the field from the PL/SQL RECORD

expr can be a variable, constant, literal, or function

call, but not a database column.

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć30

Assigning Values to Variables continued

To assign a value into a variable from the database, use a SELECT or

FETCH statement These topics are covered in later lessons

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć32

Class Management Note:

This is an example of an anonymous block of code

Assigning Values to Variables continued

Class Exercise

Evaluate the PL/SQL block on the opposite page Determine each of the followingvalues according to the rules of scoping

1. The value of V_WEIGHT within the sub-block

2. The value of V_NEW_LOCN within the sub-block

3. The value of V_WEIGHT within the main block

4. The value of V_MESSAGE within the main block

5. The value of V_NEW_LOCN within the main block

Class Management Note:

Answers:

1 V_WEIGHT = 2 (a number datatype)

2 V_NEW_LOCN = ‘Western Europe’ (a character string)

3 V_WEIGHT = 601 (a number datatype)

4 V_MESSAGE = ‘Product 10012 is in stock’ (a character string)

5 V_NEW_LOCN is illegal since it is not visible outside the sub-block

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć34

Assigning Values to Variables continued

Validate an employee number if it contains a value

Class Management Note:

Note for page 21-36

Additionally, PL/SQL provides two further functions for error reportingcalled SQLCODE and SQLERRM, which will be covered in later lesson

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć36

Assigning Values to Variables continued

Functions

Most of the functions available in SQL are also valid in PL/SQL expressions:

D Single-row number functions

D Single-row character functions

D Datatype conversion functions

D Date functions

D Miscellaneous functions

Functions not available in procedural statements:

D GREATEST and LEAST

D Group functions: AVG, MIN, MAX, COUNT, SUM, STDDEV, and VARIANCEGroup functions apply to groups of rows in a table, and are therefore available onlywithin SQL statements in a PL/SQL block

Examples

Build the mailing address for a company

CHR(10)||v_country||CHR(10)||v_zip_code;

Convert the name to uppercase

Compute the sum of all numbers stored in the NUMBER_TABLE PL/SQL table

This example produces a compile error.

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć38

Assigning Values to Variables continued

Datatype Conversion

Within an expression, you should make sure that datatypes are the same If mixeddatatypes occur in the same expression, you should use the appropriate conversionfunction from the list below to convert the data

Syntax

TO_CHAR (value, fm)

TO_DATE (value, fm)

TO_NUMBER (value, fm)

where: value is a character string, number, or date

fm is the format model used to convert value.

Examples

Store a value that is composed of the user name and today’s date This code causes a

syntax error.

To correct the error, convert SYSDATE to a character string with the TO_CHARconversion function

Introduction to Oracle: SQL and PL/SQL Using Procedure Builder 21Ć40

Assigning Values to Variables continued

Referencing NonĆPL/SQL Variables

You can reference variables declared in the host or calling environment in PL/SQLstatements, unless the statement is within a procedure, function, or package Thisincludes host language variables declared in precompiler programs, screen fields in aDeveloper/2000 Forms application, and SQL*Plus bind variables

To reference host variables, you must prefix the references with a colon (:) to

distinguish them from declared PL/SQL variables

Example

Store the annual salary in a SQL*Plus global variable