PL/SQL Tutorial | PL/SQL Explicit Cursors

Explicit cursors

 

Explicit cursors are programmer defined cursors for gaining more control over the context area. An explicit cursor should be defined in the declaration section of the PL/SQL Block. It is created on a SELECT Statement which returns more than one row.
The syntax for creating an explicit cursor is :

CURSOR cursor_name IS select_statement;

Working with an explicit cursor involves four steps:

• Declaring the cursor for initializing in the memory
• Opening the cursor for allocating memory
• Fetching the cursor for retrieving data
• Closing the cursor to release allocated memory

Declaring the Cursor

Declaring the cursor defines the cursor with a name and the associated SELECT statement. For example:
CURSOR c_customers IS SELECT id, name, address FROM customers;

Opening the Cursor

Opening the cursor allocates memory for the cursor and makes it ready for fetching the rows returned by the SQL statement into it. For example, we will open above-defined cursor as follows:
OPEN c_customers;

Fetching the Cursor

Fetching the cursor involves accessing one row at a time. For example we will fetch rows from the above-opened cursor as follows:
FETCH c_customers INTO c_id, c_name, c_addr;

Closing the Cursor

Closing the cursor means releasing the allocated memory. For example, we will close above-opened cursor as follows:
CLOSE c_customers;

Example:

Following is a complete example to illustrate the concepts of explicit cursors:

DECLARE c_id customers.id%type; 
c_name customers.name%type; 
c_addr customers.address%type; 
CURSOR c_customers is SELECT id, name, address FROM customers;  
BEGIN OPEN c_customers; 
LOOP FETCH c_customers into c_id, c_name, c_addr; 
EXIT WHEN c_customers%notfound; 
dbms_output.put_line(c_id || ' ' || c_name || ' ' || c_addr); 
END LOOP; 
CLOSE c_customers;  
END; / 

When the above code is executed at SQL prompt, it produces the following result:
1 Ramesh Ahmedabad 
2 Khilan Delhi 
3 kaushik Kota 
4 Chaitali Mumbai 
5 Hardik Bhopal 
6 Komal MP 

 PL/SQL procedure successfully completed.

PL/SQL Tutorial | PL/SQL Cursors

What are 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 in PL/SQL:

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

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.

%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

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.

%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.

  
 

PL/SQL Tutorial | PL/SQL Conditional Statements

Iterative Statements in PL/SQL

 

Iterative control Statements are used when we want to repeat the execution of one or more statements for specified number of times.

There are three types of loops in PL/SQL:

• Simple Loop
• While Loop
• For Loop 

   

General Syntax to write a Simple Loop is

  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.

PL/SQL Tutorial | PL/SQL Conditional Statements

Conditional Statements in PL/SQL

 

As the name implies, PL/SQL supports programming language features like conditional statements, iterative statements.
The programming constructs are similar to how you use in programming languages like Java and C++.

In this section I will provide you syntax of how to use conditional statements in PL/SQL programming.

Conditional Statements in PL/SQL IF THEN ELSE STATEMENT 1) IF condition THEN  statement 1; ELSE  statement 2; END IF;   2) IF condition 1 THEN  statement 1;  statement 2; ELSIF condtion2 THEN  statement 3; ELSE  statement 4; END IF

3)

IF condition 1 

THEN 

 statement 1; 

 statement 2; 

ELSIF condtion2 THEN 

 statement 3; 

ELSE 

 statement 4; 

END IF;

4)

IF condition1 THEN 

ELSE 

 IF condition2 THEN 

 statement1; 

 END IF; 

ELSIF condition3 THEN 

  statement2; 

END IF;

 

PL/SQL Tutorial | PL/SQL Records

PL/SQL Records

What are 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;

If all the fields of a record are based on the columns of a table, we can declare the record as follows:

record_name table_name%ROWTYPE;

For example, the above declaration of employee_rec can as follows:

DECLARE  employee_rec employee%ROWTYPE;

The advantages of declaring the record as a ROWTYPE are:
1)  You do not need to explicitly declare variables for all the columns in a table.
2) If you alter the column specification in the database table, you do not need to update the code.
The disadvantage of declaring the record as a ROWTYPE is:
1) When u create a record as a ROWTYPE, fields will be created for all the columns in the table and memory will be used to create the datatype for all the fields. So use ROWTYPE only when you are using all the columns of the table in the program.
NOTE: When you are creating a record, you are just creating a datatype, similar to creating a variable. You need to assign values to the record to use them.
The following table consolidates the different ways in which you can define and declare a pl/sql record.

Syntax	Usage
TYPE record_type_name IS RECORD (column_name1 datatype, column_name2 datatype, ...);	Define a composite datatype, where each field is scalar.
col_name table_name.column_name%type;	Dynamically define the datatype of a column based on a database column.
record_name record_type_name;	Declare a record based on a user-defined type.
record_name table_name%ROWTYPE;	Dynamically declare a record based on an entire row of a table. Each column in the table corresponds to a field in the record.

Passing Values To and From a Record

When you assign values to a record, you actually assign values to the fields within it.
The General Syntax to assign a value to a column within a record direclty is:

record_name.col_name := value;

If you used %ROWTYPE to declare a record, you can assign values as shown:

record_name.column_name := value;

We can assign values to records using SELECT Statements as shown:

SELECT col1, col2 INTO record_name.col_name1, record_name.col_name2 FROM table_name [WHERE clause];

If %ROWTYPE is used to declare a record then you can directly assign values to the whole record instead of each columns separately. In this case, you must SELECT all the columns from the table into the record as shown:

SELECT * INTO record_name FROM table_name [WHERE clause];

Lets see how we can get values from a record.
The General Syntax to retrieve a value from a specific field into another variable is:

var_name := record_name.col_name;

The following table consolidates the different ways you can assign values to and from a record:

Syntax	Usage
record_name.col_name := value;	To directly assign a value to a specific column of a record.
record_name.column_name := value;	To directly assign a value to a specific column of a record, if the record is declared using %ROWTYPE.
SELECT col1, col2 INTO record_name.col_name1, record_name.col_name2 FROM table_name [WHERE clause];	To assign values to each field of a record from the database table.
SELECT * INTO record_name FROM table_name [WHERE clause];	To assign a value to all fields in the record from a database table.
variable_name := record_name.col_name;	To get a value from a record column and assigning it to a variable.

PL/SQL Tutorial - PL/SQL Constants

PL/SQL Constants

As the name implies a constant is a value used in a PL/SQL Block that remains unchanged throughout the program. 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 Tutorial | PL/SQL Variables

PL/SQL Placeholders

Placeholders are temporary storage area. PL/SQL Placeholders can be any of Variables, Constants and Records. Oracle defines placeholders to store data temporarily, which are used to manipulate data during the execution of a PL SQL block.

Define PL/SQL Placeholders

Depending on the kind of data you want to store, you can define placeholders with a name and a datatype. Few of the datatypes used to define placeholders are as given below.
Number (n,m) , Char (n) , Varchar2 (n) , Date , Long , Long raw, Raw, Blob, Clob, Nclob, Bfile

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

These are placeholders that store the values that can ch

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 valueis 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.
ange through the PL/SQL Block.

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> /

      

 

Advantages of PL/SQL

These are the Advantages of PL/SQL

 

• Block Structures: PL SQL consists of blocks of code, which can be nested within each other. Each block forms a unit of a task or a logical module. PL/SQL Blocks can be stored in the database and reused.


•  Procedural Language Capability: PL SQL consists of procedural language constructs such as conditional statements (if else statements) and loops like (FOR loops).


• Error Handling: PL/SQL handles errors or exceptions effectively during the execution of a PL/SQL program. Once an exception is caught, specific actions can be taken depending upon the type of the exception or it can be displayed to the user with a message.  
•  Better Performance: PL SQL engine processes multiple SQL statements simultaneously as a single block, thereby reducing network traffic. 

PL/SQL : A Simple PL/SQL Block.

Each PL/SQL program consists of SQL and PL/SQL statements which from a PL/SQL block.

Add caption

 

PL/SQL Block consists of three sections:

• The Declaration section (optional).
• The Execution section (mandatory).
• The Exception Handling (or Error) section (optional).

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. Placeholders may be any of Variables, Constants and Records, which stores data temporarily. Cursors are also declared in this 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.

How a Sample PL/SQL Block Looks

DECLARE      Variable declaration BEGIN      Program Execution EXCEPTION      Exception handling END;

SQL Command Categories

SQL commands are grouped into four major categories depending on their functionality. They are as follows:

Data Definition Language (DDL)

These SQL commands are used for creating, modifying, and dropping the structure of database objects. The commands are CREATE, ALTER, DROP, RENAME, and TRUNCATE.

Data Manipulation Language (DML)

These SQL commands are used for storing, retrieving, modifying, and deleting data. These commands are SELECT, INSERT, UPDATE, and DELETE.

Transaction Control Language (TCL)

These SQL commands are used for managing changes affecting the data. These commands are COMMIT, ROLLBACK, and SAVEPOINT.

Data Control Language (DCL)

These SQL commands are used for providing security to database objects. These commands are GRANT and REVOKE.

 

What is PL/SQL?

PL/SQL stands for Procedural Language extension of SQL.

 

PL/SQL is a combination of SQL along with the procedural features of programming languages.

It was developed by Oracle Corporation in the early 90’s to enhance the capabilities of SQL.

The PL/SQL Engine:

Oracle uses a PL/SQL engine to processes the PL/SQL statements. A PL/SQL code can be stored in the client system (client-side) or in the database (server-side).

About This PL SQL Programming Tutorial

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 language and syntax better.
Even though the programming concepts discussed in this tutorial are specific to Oracle PL SQL. The concepts like cursors, functions and stored procedures can be used in other database systems like Sybase , Microsoft SQL server etc, with some change in SQL syntax. 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.