You can also add "complex logic" to your computed columns via CASE expressions.  This might offer performance benefits over a UDF.

Just as a really simple example:

CREATE TABLE test (

 i INT NOT NULL PRIMARY KEY,

 j INT NULL,

 k AS (CASE

 WHEN j < 0 THEN i

 WHEN j IS NULL THEN 0

 ELSE (i * j)

 END)

)

GO

INSERT INTO test (i, j)

SELECT 0, 1

UNION SELECT 1, 1

UNION SELECT 2, 3

UNION SELECT 4, NULL

UNION SELECT 10, -1

GO

SELECT *

FROM test

In the sample, k is computed based on the value of j.  If j is negative then k = i, if j is NULL then k = 0, otherwise k = i * j.  Really simple and not really all that useful of an example, but it's a pretty powerful concept.

You can also add an index to a computed column with a CASE expression:

CREATE  INDEX IX_test ON test(k)

GO