Explicit Cursors

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.

The 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.


How to use Explicit Cursor?

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.

How to access an Explicit Cursor?
These are the three steps in accessing the cursor.
1) Open the cursor.
2) Fetch the records in the cursor one at a time.
3) 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;OR

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;

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;

In the above example, first we are creating a record ‘emp_rec’ of the same structure as of table ‘emp_tbl’ in line no 2. We can also create a record with a cursor by replacing the table name with the cursor name. Second, we are declaring a cursor ‘emp_cur’ from a select query in line no 3 - 6. Third, we are opening the cursor in the execution section in line no 8. Fourth, we are fetching the cursor to the record in line no 9. Fifth, we are displaying the first_name and last_name of the employee in the record emp_rec in line no 10. Sixth, we are closing the cursor in line no 11.

What are 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.

What are 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:
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.


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.

Iterative Statements in PL/SQL

Iterative Statements in PL/SQL

An iterative control Statements are used when we want to repeat the execution of one or more statements for specified number of times. These are similar to those in

There are three types of loops 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.

The 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
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.

Conditional Statements in PL/SQL

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.

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 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.

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.

The 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 Placeholders

PL/SQL Placeholders
Placeholders are temporary storage area. 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.

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.

The 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 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.
DECLAREsalary 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.

DECLAREsalary 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 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> /