A constant holds a value that once declared, does not change in the prouram. Declaring a constant is similar to declaring a variable—eve-lit that you have to add he keyword 'constant' and immediately assign a value to it. Thereafter, no further assignments to the constant are possible. This constant can be used anywhere in the code.
For example:
C_Max CONSTANT NUMBER: = 500;
C_Exp CONSTANT VARCHAR2: = `MIN5'
Minval CONSTANT INTEGER(2): = 22;
Using Logical Comparisons in PL/SQL
PL/SQL supports the comparison between variables and constants in SQL and PL/SQL statements. These comparisons, often called Boolean expressions, generally consist of simple expressions separated by relational operators (<, >, =, < >, >=, <= that can be connected by logical operators (AND, OR, NOT). A Boolean expression will always evaluate to TRUE, FALSE or NULL.
For example, (A <7 b >=-: a 2*a = b) ---> Numeric comparisons
(Namel > name2, namel = 'Jones', Product_type != 'COMPUTER')-> Character comparisons
(birthday < '05-Jul-59', hiredate <=sysdate) ---> date comparisons
