Functions returning a number, text, or date Value

A function is a subprogram that can take parameters and return a single value. A function has two parts:

1. the specification and
2. the body.

The specification (spec for short) begins with the keyword FUNCTION and ends with the RETURN clause, which specifies the datatype of the return value. Parameter declarations are optional and functions that take no parameters are written without parentheses. The function body begins with the keyword IS (or AS) and ends with the keyword END followed by an optional function name.

The function body has three parts:

1. an optional declarative part,
2. an executable part, and
3. an optional exception-handling part.

The declarative part contains declarations of

1. types,
2. cursors,
3. constants,
4. variables,
5. exceptions, and
6. subprograms.

These items are local and cease to exist when you exit the function. The executable part contains statements that assign values, control execution, and manipulate data. The exception-handling part contains handlers that deal with exceptions raised during execution. Note that the function declaration in a PL/SQL block or package is not the same as creating a function in SQL.

Here is an example of an Oracle PL/SQL function that returns both a number and text. This function will take an input parameter, perform a calculation, and return a number along with a descriptive text.

CREATE OR REPLACE FUNCTION calculate_area(radius IN NUMBER) 
RETURN VARCHAR2 IS
  pi CONSTANT NUMBER := 3.14159;
  area NUMBER;
  result_text VARCHAR2(100);
BEGIN
  area := pi * radius * radius;
  result_text := 'The area of the circle with radius ' || TO_CHAR(radius) || ' is ' || TO_CHAR(area);
  RETURN result_text;
END calculate_area;
/

In this function, calculate_area, we declare a constant pi and two variables area and result_text. The function takes one input parameter, radius, which is of type NUMBER. The function is declared to return a VARCHAR2 type, which is a string of variable length.
The BEGIN and END keywords enclose the body of the function. In the body, we calculate the area of a circle using the formula pi * radius * radius and assign it to the area variable. We then construct a string result_text that includes both the input radius and the calculated area. This string is returned by the function.
To call this function, you would use a SQL statement like this:

SELECT calculate_area(5) FROM dual;

This would return: "The area of the circle with radius 5 is 78.53975".