Reusing lines of code

I have a very large select statement. I was hoping to somehow take out the repeated code that is
used and store it somewhere and access it through something like an include statement that is
used in other lanuages.
For example, I have a select statement that creates a column and then that code is expanded upon and
stored as another column.
SELECT (code here) AS column1,
(code here)*5 AS column2,
(code here)/345 AS column3
FROM table

I have the same code written multiple times. Is there a way to store it somewhere else and include it
in the statement instead of writing it multiple times?
(code here) is usually a case statement with multiple whens.

You could create it as a view, which will make it available for other procedures too.
If this is for use within one procedure, and for one query only, you could use the WITH syntax (SQL 2005 and up) or a derived table (a query statement between brackets, with an alias, in your FROM clause).
If your procedure needs to access this same data more than once, then you could store the data in a table variable, or in a temporary table that you create within the procedure.

So this is an example of my code that I reuse over in the select statement. I do not believe that I can create a view with it.
CONVERT(DECIMAL(10,1),
(
CASE WHEN DATEDIFF(DAY, AMD, AFD)>=0 THEN NULL
ELSE
(
CAST(
(
(CASE
WHEN FBI='1' THEN (CASE
WHEN DATEDIFF(DAY, AED, AFD)>=0 THEN 0
ELSE ABS(ROUND((DATEDIFF(DAY, AED, AFD)/365.25),10))
END
)
WHEN FBI='2' AND (CASE
WHEN ATS='A' THEN 'TRUE'
WHEN ATS='B' THEN 'TRUE'
WHEN ATS='C' THEN 'FALSE'
ELSE NULL
END)='TRUE' THEN (CASE
WHEN DATEDIFF(DAY, AMD, AFD)>=0 THEN NULL
ELSE CAST((ABS(ROUND((DATEDIFF(DAY, AMD, AFD)/365.25),10))) AS VARCHAR(50))
END
)
WHEN FBI='3' THEN (1IT)
WHEN FBI='4' THEN (2IT)
--
END)
+
(CASE
WHEN TTMITB='1' THEN (CASE
WHEN 1IT >= 2IT THEN 1IT
WHEN 2IT >= 1IT THEN 2IT
END
)
ELSE 0
END)
) AS VARCHAR(20))
)
END
)
)

The whole thing that starts with CONVERT, all the way down to the final closing brackets, is a single expression.
In a query, instead of a column name, you can specify an expression.
A view is always a SELECT query. So in your view, you can specify the whole expression with an alias. This alias will be the column name when you access the view.

I learned that using a common table expression would help in my case.