Variables and constants are initialized every time a block or subprogram is entered.
By default, variables are initialized to NULL. So, unless you expressly initialize a variable, its value is undefined, as the following example shows:
count INTEGER;
count := count + 1; -- assigns a null to count

The expression on the right of the assignment operator yields NULL because count is null. To avoid unexpected results, never reference a variable before you assign it a value.

You can use assignment statements to assign values to a variable. For example, the following statement assigns a new value to the variable bonus, overwriting its old value:
bonus := salary * 0.15;
The expression following the assignment operator can be arbitrarily complex, but it must yield a datatype that is the same as or convertible to the datatype of the variable.

Boolean Values
Only the values TRUE, FALSE, and NULL can be assigned to a Boolean variable. For example, given the declaration
the following statements are legal:
done := FALSE;

When applied to an expression, the relational operators return a Boolean value. So, the following assignment is legal:
done := (count > 500);
Expressions and Comparisons

Database Values
You can use the SELECT statement to have Oracle assign values to a variable. For each item in the select list, there must be a corresponding, type-compatible variable in the INTO list. An example follows:
my_empno emp.empno%TYPE;
my_ename emp.ename%TYPE;
wages NUMBER(7,2);
SELECT ename, sal + comm
INTO last_name, wages FROM emp
WHERE empno = emp_id;

However, you cannot select column values into a Boolean variable.

Quick notes -Assignment
1. := (ASSIGNMENT ) whereas = (VALUE EQUALITY)
2. The datatype of the left and right hand side of an assignment must be the same or implicitly convertible to each other.                    
For ex. , N:=‘7’ is legal because number may be implicitly converted to char.
3. Column or table reference are not allowed on either side of an assignment operator( : = ).
                SCOTT.EMP.EMPNO := 1234;
                  location := dept.loc.;    

Above two are incorrect.

Oracle Applications Fusion Cloud - Inventory

Oracle Cloud/Fusion Procurement training will help you develop the fundamental skills required to set up and use the Procurement module. This training covers all the tasks, setups, forms and reports used in Procurement and related modules