Data Warehouse Concepts: PL-SQL Tutorial

Introduction

This Oracle PL SQL tutorial teaches you the basics of database programming in PL/SQL with appropriate PL/SQL tutorials with examples. You can use these free online tutorials as your guide to practice, learn, for training, or reference while programming with PL SQL. I will be making more Oracle PL SQL programming tutorials as often as possible to share my knowledge in PL SQL and help you in learning PL SQL better.

PL-SQL :

PL-SQL is a combination of SQL and procedural features of programming languages. It was developed by Oracle Corporation in early 90’s to enhance to capabilities of SQL. PL-SQL can be stored in client side or in data base server itself and it process by using PL-SQL Engine.

This PL/SQL tutorial will be growing regularly; let us know if any topic related to PL SQL needs to be added or you can also share your knowledge on PL SQL with us. Lets share our knowledge about PL SQL with others.

Structure of PL-SQL :

The Structure of any PL-SQL statement has three blocks as mentioned in the below :

DECLARE
Variable declaration

BEGIN
Program Execution

EXCEPTION
Exception handling
END;

Declaration Section:

The Declaration section of a PL/SQL Block starts with the reserved keyword DECLARE. This section is optional and is used to declare any placeholders like variables, constants, records and cursors, which are used to manipulate data in the execution section.

Execution Section:

The Execution section of a PL/SQL Block starts with the reserved keyword BEGIN and ends with END. This is a mandatory section and is the section where the program logic is written to perform any task. The programmatic constructs like loops, conditional statement and SQL statements form the part of execution section.

Exception Section:

The Exception section of a PL/SQL Block starts with the reserved keyword EXCEPTION. This section is optional. Any errors in the program can be handled in this section, so that the PL/SQL Blocks terminates gracefully. If the PL/SQL Block contains exceptions that cannot be handled, the Block terminates abruptly with errors.

Every statement in the above three sections must end with a semicolon ; . PL/SQL blocks can be nested within other PL/SQL blocks. Comments can be used to document code.

DECLARE

message varchar2(20):= 'Hello, World!';

BEGIN

dbms_output.put_line(message);

END;

The end; line signals the end of the PL/SQL block. To run the code from SQL command line, you may need to type / at the beginning of the first blank line after the last line of the code. When the above code is executed at SQL prompt, it produces the following result:

Hello World

PL/SQL procedure successfully completed.

PL/SQL Variables

These are placeholders that store the values that can change through the PL/SQL Block.

General Syntax to declare a variable is

variable_name datatype [NOT NULL := value ];

variable_name is the name of the variable. datatype is a valid PL/SQL datatype. NOT NULL is an optional specification on the variable.

value or DEFAULT value is also an optional specification, where you can initialize a variable. Each variable declaration is a separate statement and must be terminated by a semicolon. For example, if you want to store the current salary of an employee, you can use a variable.

DECLARE

salary number (6);

* “salary” is a variable of datatype number and of length 6.

When a variable is specified as NOT NULL, you must initialize the variable when it is declared.

For example: The below example declares two variables, one of which is a not null.

DECLARE

salary number(4);

dept varchar2(10) NOT NULL := “HR Dept”;

The value of a variable can change in the execution or exception section of the PL/SQL Block. We can assign values to variables in the two ways given below.

1) We can directly assign values to variables.
The General Syntax is:

variable_name:= value;

2) We can assign values to variables directly from the database columns by using a SELECT.. INTO statement. The General Syntax is:

SELECT column_name

INTO variable_name

FROM table_name

[WHERE condition];

Example: The below program will get the salary of an employee with id '1116' and display it on the screen.

DECLARE

var_salary number(6);

var_emp_id number(6) = 1116;

BEGIN

SELECT salary

INTO var_salary

FROM employee

WHERE emp_id = var_emp_id;

dbms_output.put_line(var_salary);

dbms_output.put_line('The employee '

|| var_emp_id || ' has salary ' || var_salary);

END;

NOTE: The backward slash '/' in the above program indicates to execute the above PL/SQL Block.

Scope of PS/SQL Variables

PL/SQL allows the nesting of Blocks within Blocks i.e, the Execution section of an outer block can contain inner blocks. Therefore, a variable which is accessible to an outer Block is also accessible to all nested inner Blocks. The variables declared in the inner blocks are not accessible to outer blocks. Based on their declaration we can classify variables into two types.

Local variables - These are declared in a inner block and cannot be referenced by outside Blocks.

Global variables - These are declared in a outer block and can be referenced by its itself and by its inner blocks.

For Example: In the below example we are creating two variables in the outer block and assigning thier product to the third variable created in the inner block. The variable 'var_mult' is declared in the inner block, so cannot be accessed in the outer block i.e. it cannot be accessed after line 11. The variables 'var_num1' and 'var_num2' can be accessed anywhere in the block.

1> DECLARE

2> var_num1 number;

3> var_num2 number;

4> BEGIN

5> var_num1 := 100;

6> var_num2 := 200;

7> DECLARE

8> var_mult number;

9> BEGIN

10> var_mult := var_num1 * var_num2;

11> END;

12> END;

13> /

PL/SQL Constants

A constant is a user-defined literal value. You can declare a constant and use it instead of actual value. For example:

If you want to write a program which will increase the salary of the employees by 25%, you can declare a constant and use it throughout the program. Next time when you want to increase the salary again you can change the value of the constant which will be easier than changing the actual value throughout the program.

General Syntax to declare a constant is:

constant_name CONSTANT datatype := VALUE;

constant_name is the name of the constant i.e. similar to a variable name. The word CONSTANT is a reserved word and ensures that the value does not change. VALUE - It is a value which must be assigned to a constant when it is declared. You cannot assign a value later.

For example, to declare salary_increase, you can write code as follows:

DECLARE

salary_increase CONSTANT number (3) := 10;

You must assign a value to a constant at the time you declare it. If you do not assign a value to a constant while declaring it and try to assign a value in the execution section, you will get a error. If you execute the below Pl/SQL block you will get error.

DECLARE

salary_increase CONSTANT number(3);

BEGIN

salary_increase := 100;

dbms_output.put_line (salary_increase);

END;

PL/SQL Records

Records are another type of datatypes which oracle allows to be defined as a placeholder. Records are composite datatypes, which means it is a combination of different scalar datatypes like char, varchar, number etc. Each scalar data types in the record holds a value. A record can be visualized as a row of data. It can contain all the contents of a row.

Declaring a record:

To declare a record, you must first define a composite datatype; then declare a record for that type.

The General Syntax to define a composite datatype is:

TYPE record_type_name IS RECORD

(first_col_name column_datatype,

second_col_name column_datatype, ...);

record_type_name – it is the name of the composite type you want to define.
first_col_name, second_col_name, etc.,- it is the names the fields/columns within the record.
column_datatype defines the scalar datatype of the fields.

There are different ways you can declare the datatype of the fields.

1) You can declare the field in the same way as you declare the fieds while creating the table.
2) If a field is based on a column from database table, you can define the field_type as follows:

col_name table_name.column_name%type;

By declaring the field datatype in the above method, the datatype of the column is dynamically applied to the field. This method is useful when you are altering the column specification of the table, because you do not need to change the code again.

NOTE: You can use also %type to declare variables and constants.

The General Syntax to declare a record of a uer-defined datatype is:

record_name record_type_name;

The following code shows how to declare a record called employee_rec based on a user-defined type.

DECLARE
TYPE employee_type IS RECORD
(employee_id number(5),
employee_first_name varchar2(25),
employee_last_name employee.last_name%type,
employee_dept employee.dept%type);
employee_salary employee.salary%type;
employee_rec employee_type;

Conditional Statements in PL/SQL

IF THEN ELSE STATEMENT

IF condition

THEN

statement 1;

ELSE

statement 2;

END IF;

IF condition 1

THEN

statement 1;

statement 2;

ELSIF condtion2 THEN

statement 3;

ELSE

statement 4;

END IF

IF condition 1

THEN

statement 1;

statement 2;

ELSIF condtion2 THEN

statement 3;

ELSE

statement 4;

END IF;

IF condition1 THEN

ELSE

IF condition2 THEN

statement1;

END IF;

ELSIF condition3 THEN

statement2;

END IF;

Iterative Statements in PL/SQL

• Simple Loop
• While Loop
• For Loop

1) Simple Loop

A Simple Loop is used when a set of statements is to be executed at least once before the loop terminates. An EXIT condition must be specified in the loop, otherwise the loop will get into an infinite number of iterations. When the EXIT condition is satisfied the process exits from the loop.

General Syntax to write a Simple Loop is

LOOP

statements;

EXIT;

{or EXIT WHEN condition;}

END LOOP;

These are the important steps to be followed while using Simple Loop.

1) Initialise a variable before the loop body.
2) Increment the variable in the loop.
3) Use a EXIT WHEN statement to exit from the Loop. If you use a EXIT statement without WHEN condition, the statements in the loop is executed only once.

2) While Loop

A WHILE LOOP is used when a set of statements has to be executed as long as a condition is true. The condition is evaluated at the beginning of each iteration. The iteration continues until the condition becomes false.

The General Syntax to write a WHILE LOOP is:

WHILE <condition>

LOOP statements;

END LOOP;

Important steps to follow when executing a while loop:

1) Initialise a variable before the loop body.
2) Increment the variable in the loop.
3) EXIT WHEN statement and EXIT statements can be used in while loops but it's not done oftenly.

3) FOR Loop

A FOR LOOP is used to execute a set of statements for a predetermined number of times. Iteration occurs between the start and end integer values given. The counter is always incremented by 1. The loop exits when the counter reachs the value of the end integer.

The General Syntax to write a FOR LOOP is:

FOR counter IN val1..val2

LOOP statements;

END LOOP;

val1 - Start integer value.

val2 - End integer value.

Important steps to follow when executing a while loop:

1) The counter variable is implicitly declared in the declaration section, so it's not necessary to declare it explicity.
2) The counter variable is incremented by 1 and does not need to be incremented explicitly.
3) EXIT WHEN statement and EXIT statements can be used in FOR loops but it's not done oftenly.

NOTE: The above Loops are explained with a example when dealing with Explicit Cursors

Cursors

A cursor is a temporary work area created in the system memory when a SQL statement is executed. A cursor contains information on a select statement and the rows of data accessed by it.

This temporary work area is used to store the data retrieved from the database, and manipulate this data. A cursor can hold more than one row, but can process only one row at a time. The set of rows the cursor holds is called the active set.

There are two types of cursors :

Implicit cursors

These are created by default when DML statements like, INSERT, UPDATE, and DELETE statements are executed. They are also created when a SELECT statement that returns just one row is executed.

Explicit cursors

They must be created when you are executing a SELECT statement that returns more than one row. Even though the cursor stores multiple records, only one record can be processed at a time, which is called as current row. When you fetch a row the current row position moves to next row.

Both implicit and explicit cursors have the same functionality, but they differ in the way they are accessed.

Implicit Cursors: Application

When you execute DML statements like DELETE, INSERT, UPDATE and SELECT statements, implicit statements are created to process these statements.

Oracle provides few attributes called as implicit cursor attributes to check the status of DML operations. The cursor attributes available are %FOUND, %NOTFOUND, %ROWCOUNT, and %ISOPEN.

For example, When you execute INSERT, UPDATE, or DELETE statements the cursor attributes tell us whether any rows are affected and how many have been affected.
When a SELECT... INTO statement is executed in a PL/SQL Block, implicit cursor attributes can be used to find out whether any row has been returned by the SELECT statement. PL/SQL returns an error when no data is selected.

The status of the cursor for each of these attributes are defined in the below table.

Attributes	Return Value	Example
%FOUND	The return value is TRUE, if the DML statements like INSERT, DELETE and UPDATE affect at least one row and if SELECT ….INTO statement return at least one row.	SQL%FOUND
%FOUND	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE do not affect row and if SELECT….INTO statement do not return a row.	SQL%FOUND
%NOTFOUND	The return value is FALSE, if DML statements like INSERT, DELETE and UPDATE at least one row and if SELECT ….INTO statement return at least one row.	SQL%NOTFOUND
%NOTFOUND	The return value is TRUE, if a DML statement like INSERT, DELETE and UPDATE do not affect even one row and if SELECT ….INTO statement does not return a row.	SQL%NOTFOUND
%ROWCOUNT	Return the number of rows affected by the DML operations INSERT, DELETE, UPDATE, SELECT	SQL%ROWCOUNT

For Example: Consider the PL/SQL Block that uses implicit cursor attributes as shown below:

DECLARE var_rows number(5);

BEGIN

UPDATE employee

SET salary = salary + 1000;

IF SQL%NOTFOUND THEN

dbms_output.put_line('None of the salaries where updated');

ELSIF SQL%FOUND THEN

var_rows := SQL%ROWCOUNT;

dbms_output.put_line('Salaries for ' || var_rows || 'employees are updated');

END IF;

END;

In the above PL/SQL Block, the salaries of all the employees in the ‘employee’ table are updated. If none of the employee’s salary are updated we get a message 'None of the salaries where updated'. Else we get a message like for example, 'Salaries for 1000 employees are updated' if there are 1000 rows in ‘employee’ table.

Explicit Cursors

An explicit cursor is defined in the declaration section of the PL/SQL Block. It is created on a SELECT Statement which returns more than one row. We can provide a suitable name for the cursor.

General Syntax for creating a cursor is as given below:

CURSOR cursor_name IS select_statement;

cursor_name – A suitable name for the cursor.
select_statement – A select query which returns multiple rows.

There are four steps in using an Explicit Cursor.

DECLARE the cursor in the declaration section.
OPEN the cursor in the Execution Section.
FETCH the data from cursor into PL/SQL variables or records in the Execution Section.
CLOSE the cursor in the Execution Section before you end the PL/SQL Block.

1) Declaring a Cursor in the Declaration Section:

DECLARE

CURSOR emp_cur IS

SELECT *

FROM emp_tbl

WHERE salary > 5000;

In the above example we are creating a cursor ‘emp_cur’ on a query which returns the records of all the employees with salary greater than 5000. Here ‘emp_tbl’ in the table which contains records of all the employees.

2) Accessing the records in the cursor:

Once the cursor is created in the declaration section we can access the cursor in the execution
section of the PL/SQL program.

These are the three steps in accessing the cursor.

Open the cursor..
Fetch the records in the cursor one at a time.
Close the cursor.

General Syntax to open a cursor is:

OPEN cursor_name;

General Syntax to fetch records from a cursor is:

FETCH cursor_name INTO record_name;

FETCH cursor_name INTO variable_list;

General Syntax to close a cursor is:

CLOSE cursor_name;

When a cursor is opened, the first row becomes the current row. When the data is fetched it is copied to the record or variables and the logical pointer moves to the next row and it becomes the current row. On every fetch statement, the pointer moves to the next row. If you want to fetch after the last row, the program will throw an error. When there is more than one row in a cursor we can use loops along with explicit cursor attributes to fetch all the records.

Points to remember while fetching a row:

· We can fetch the rows in a cursor to a PL/SQL Record or a list of variables created in the PL/SQL Block.
· If you are fetching a cursor to a PL/SQL Record, the record should have the same structure as the cursor.
· If you are fetching a cursor to a list of variables, the variables should be listed in the same order in the fetch statement as the columns are present in the cursor.

General Form of using an explicit cursor is:

DECLARE

variables;

records;

create a cursor;

BEGIN

OPEN cursor;

FETCH cursor;

process the records;

CLOSE cursor;

END;

Explicit Cursor, Lets Look at the example below

Example 1:

1> DECLARE

2> emp_rec emp_tbl%rowtype;

3> CURSOR emp_cur IS

4> SELECT *

5> FROM

6> WHERE salary > 10;

7> BEGIN

8> OPEN emp_cur;

9> FETCH emp_cur INTO emp_rec;

10> dbms_output.put_line (emp_rec.first_name || ' ' || emp_rec.last_name);

11> CLOSE emp_cur;

12> END;

Explicit Cursor Attributes :

Oracle provides some attributes known as Explicit Cursor Attributes to control the data processing while using cursors. We use these attributes to avoid errors while accessing cursors through OPEN, FETCH and CLOSE Statements.

When does an error occur while accessing an explicit cursor?

a) When we try to open a cursor which is not closed in the previous operation.
b) When we try to fetch a cursor after the last operation.

These are the attributes available to check the status of an explicit cursor.

Attributes	Return values	Example
%FOUND	TRUE, if fetch statement returns at least one row.	Cursor_name%FOUND
	FALSE, if fetch statement doesn’t return a row.
%NOTFOUND	TRUE, , if fetch statement doesn’t return a row.	Cursor_name%NOTFOUND
	FALSE, if fetch statement returns at least one row.
%ROWCOUNT	The number of rows fetched by the fetch statement	Cursor_name%ROWCOUNT
	If no row is returned, the PL/SQL statement returns an error.
%ISOPEN	TRUE, if the cursor is already open in the program	Cursor_name%ISNAME
	FALSE, if the cursor is not opened in the program.

Using Loops with Explicit Cursors:

Oracle provides three types of cursors namely SIMPLE LOOP, WHILE LOOP and FOR LOOP. These loops can be used to process multiple rows in the cursor. Here I will modify the same example for each loops to explain how to use loops with cursors.

Cursor with a Simple Loop:

1> DECLARE

2> CURSOR emp_cur IS

3> SELECT first_name, last_name, salary FROM emp_tbl;

4> emp_rec emp_cur%rowtype;

5> BEGIN

6> IF NOT sales_cur%ISOPEN THEN

7> OPEN sales_cur;

8> END IF;

9> LOOP

10> FETCH emp_cur INTO emp_rec;

11> EXIT WHEN emp_cur%NOTFOUND;

12> dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name

13> || ' ' ||emp_cur.salary);

14> END LOOP;

15> END;

16> /

In the above example we are using two cursor attributes %ISOPEN and %NOTFOUND. In line no 6, we are using the cursor attribute %ISOPEN to check if the cursor is open, if the condition is true the program does not open the cursor again, it directly moves to line no 9. In line no 11, we are using the cursor attribute %NOTFOUND to check whether the fetch returned any row. If there is no rows found the program would exit, a condition which exists when you fetch the cursor after the last row, if there is a row found the program continues.

We can use %FOUND in place of %NOTFOUND and vice versa. If we do so, we need to reverse the logic of the program. So use these attributes in appropriate instances.

Cursor with a While Loop:

Lets modify the above program to use while loop.

1> DECLARE

2> CURSOR emp_cur IS

3> SELECT first_name, last_name, salary FROM emp_tbl;

4> emp_rec emp_cur%rowtype;

5> BEGIN

6> IF NOT sales_cur%ISOPEN THEN

7> OPEN sales_cur;

8> END IF;

9> FETCH sales_cur INTO sales_rec;

10> WHILE sales_cur%FOUND THEN

11> LOOP

12> dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name

13> || ' ' ||emp_cur.salary);

15> FETCH sales_cur INTO sales_rec;

16> END LOOP;

17> END;

18> /

In the above example, in line no 10 we are using %FOUND to evaluate if the first fetch statement in line no 9 returned a row, if true the program moves into the while loop. In the loop we use fetch statement again (line no 15) to process the next row. If the fetch statement is not executed once before the while loop the while condition will return false in the first instance and the while loop is skipped. In the loop, before fetching the record again, always process the record retrieved by the first fetch statement, else you will skip the first row.

Cursor with a FOR Loop:

When using FOR LOOP you need not declare a record or variables to store the cursor values, need not open, fetch and close the cursor. These functions are accomplished by the FOR LOOP automatically.

General Syntax for using FOR LOOP:

FOR record_name IN cusror_name

LOOP

process the row...

END LOOP;

Let’s use the above example to learn how to use for loops in cursors.

1> DECLARE

2> CURSOR emp_cur IS

3> SELECT first_name, last_name, salary FROM emp_tbl;

4> emp_rec emp_cur%rowtype;

5> BEGIN

6> FOR emp_rec in sales_cur

7> LOOP

8> dbms_output.put_line(emp_cur.first_name || ' ' ||emp_cur.last_name

9> || ' ' ||emp_cur.salary);

10> END LOOP;

11>END;

12> /

In the above example, when the FOR loop is processed a record ‘emp_rec’of structure ‘emp_cur’ gets created, the cursor is opened, the rows are fetched to the record ‘emp_rec’ and the cursor is closed after the last row is processed. By using FOR Loop in your program, you can reduce the number of lines in the program.

NOTE: In the examples given above, we are using backward slash ‘/’ at the end of the program. This indicates the oracle engine that the PL/SQL program has ended and it can begin processing the statements

Data Warehouse Concepts

Thursday, 28 November 2013

PL-SQL Tutorial

PL/SQL Records

Declaring a record:

No comments:

Post a Comment

Contributors