which to use: LIKE, =, IN... help!!!

im pulling information out of a DB2 database on an AS/400 and get different, sometimes strange results
depending on which i use. im not doing any math on any of the fields i pull in, although most of the fields hold numerical data...
but im only displaying it on a webpage... so for example lets say a customer number is 150 and i want to display records that match
that customer number:

A) SELECT CUSNO WHERE CUSNO = 150
B) SELECT CUSNO WHERE CUSNO = '150'
C) SELECT CUSNO WHERE CUSNO LIKE '150'

and then i can also do something with the "IN" keyword like:

SELECT CUSNO WHERE CUSNO IN (150)
SELECT CUSNO WHERE CUSNO IN ('150')

so which way is the "right" way? i think im being confused by whether or not to use "=" and whether or not to use single quotes...
anyone wanna shed some light on this for me?
thank you!

I'm not familiar with the particulars of DB2, but it seems like your question is in regard to standard SQL

So unless DB2 isn't following standards, here you go:

The IN is used to check for a list of values, whereas the = checks for only 1 value
SELECT CUSNO WHERE CUSNO = 150
SELECT CUSNO WHERE CUSNO IN (150, 151, 160, 215, 35000)

The quotes are used to specify a string value. Numbers don't require the quotes, but strings do
SELECT CUSNO WHERE CUSNO = 150
SELECT CUSNO WHERE LNAME = 'Smith'
SELECT CUSNO WHERE LNAME IN ('Smith', 'Jones', 'Wakefield')