PL/SQL is a combination of SQL along with the procedural features of
programming languages.
Each PL/SQL program consists of SQL and PL/SQL statements which from a 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.
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; |
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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.
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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
STATEMENT1) 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 |
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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. |
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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.
