how to use variables in sql

I'm beginner in Oracle. I'm wondering how to use variables in SQL, to make statement less complicated. For example:

SELECT      substr(nnp,instr(nnp, '*')+1, instr(nnp, '/')+1 - instr(nnp, '*')+1)FROM dav_5;

i want to make like this:

A := instr(nnp, '*')+1
B := instr(nnp, '/')+1
SELECT SUBSTR (nnp, A, B-A)
FROM dav_5

What should I do, to run this in SQL Plus?
tomaz108Asked:
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sapnamCommented:
define A = "instr(nnp, '*')+1"
define B = "instr(nnp, '/')+1"
SELECT SUBSTR (nnp, &A, &B-&A)
FROM dav_5

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sapnamCommented:
As your variable containts single quotes, you need to define them with enclosing double quotes
kamal_therockyCommented:
Hi

You need to use Define to declare variables which can be used in SQL.

you just give the comman DEFINE to see what all variable are already declared.

SQL> DEFINE A
SQL> DEFINE B
SQL> ACCEPT A CHAR PROMPT 'Enter Value of First Expression'
SQL> ACCEPT B CHAR PROMPT 'Enter value of second Expression'
SQL> SELECT SUBSTR (nnp, &A, &B-&A) FROM dav_5

kamal
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