single quote is used to write a character string or character in sql query.

but double quotes are used to print the name in sql screen.

for eg:-

select sysdate current date from dual;

current date

-----------------

24-mar-06.

in single quote oracle takes the character as a keyword.

double quote( ) is used to give user defined column alias

example:

select empno ename employee name from emp;

this will print column heading as: EMPNO employee name

where as single quote is used for passing a string

select ename||' is employee since '||hiredate from emp;

Date and character literals must be enclosed within single quotation marks (' ');