pl/sql tutorial

http://plsql-tutorial.com/plsql-procedures.htm

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.

A Simple PL/SQL Block:

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

PL/SQL Block consists of three sections:

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

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. 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.

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; 

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.

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;

 

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 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.