What is the difference between variable and Define

Question

WHAT IS THE DIFFERENCE BETWEEN
these different declarations when it is done using the keyword "variable" and "define"
and both of these are done OUTSIDE "DECLARE"
VARIABLE g_monthly_sal NUMBER
DEFINE p_annual_sal =5000
-- I understand that p_annual_sal will be taken as a CHAR.
-- ALSO IF DEFINE variable acts as macro variable, SO is it necessary to give it some value whenever we define it.
if not what value would be substituted for it?
OR does that mean whenever we want to specify data type for a bind varible we should use VARIABLE and
when we do not want to specify type we use DEFINE?
THANK YOU
Edited by: user6287828 on Feb 24, 2009 11:03 AM
Edited by: user6287828 on Feb 24, 2009 11:04 AM

Answer 1

Both are SQL*plus commands. In a real programming environment you will not use such constructs (except a few rare scripting cases).
The difference is how the construct is later used. DEFINE is more like a copy&paste string. Whereever the name of this substitution variable is found it will be pasted into the sql*plus session.
VARIABLE creates a real variable. You can change the value and if follwos the usual principles of variables (including binding).
Example can be found the docs:
from the docs
Where and How to Use Substitution Variables
You can use substitution variables anywhere in SQL and SQL*Plus commands, except as the first word entered. When SQL*Plus encounters an undefined substitution variable in a command, SQL*Plus prompts you for the value.
You can enter any string at the prompt, even one containing blanks and punctuation. If the SQL command containing the reference should have quote marks around the variable and you do not include them there, the user must include the quotes when prompted.
SQL*Plus reads your response from the keyboard, even if you have redirected terminal input or output to a file. If a terminal is not available (if, for example, you run the script in batch mode), SQL*Plus uses the redirected file.
After you enter a value at the prompt, SQL*Plus lists the line containing the substitution variable twice: once before substituting the value you enter and once after substitution. You can suppress this listing by setting the SET command variable VERIFY to OFF.
Using Bind Variables
Bind variables are variables you create in SQL*Plus and then reference in PL/SQL or SQL. If you create a bind variable in SQL*Plus, you can use the variable as you would a declared variable in your PL/SQL subprogram and then access the variable from SQL*Plus. You can use bind variables for such things as storing return codes or debugging your PL/SQL subprograms.
Because bind variables are recognized by SQL*Plus, you can display their values in SQL*Plus or reference them in PL/SQL subprograms that you run in SQL*Plus.
Creating Bind Variables
You create bind variables in SQL*Plus with the VARIABLE command. For example
VARIABLE ret_val NUMBER
This command creates a bind variable named ret_val with a datatype of NUMBER. See the VARIABLE command for more information. (To list all bind variables created in a session, type VARIABLE without any arguments.)
Referencing Bind Variables
You reference bind variables in PL/SQL by typing a colon (:) followed immediately by the name of the variable. For example
:ret_val := 1;
To change this bind variable in SQL*Plus, you must enter a PL/SQL block. For example:
BEGIN
:ret_val:=4;
END;
/