WHERE with CASE and null checking

Question

i have a problem with a CASE statement in a WHERE clause. In my query i want to get rows which are before a date. The date is check on 3 columns. date1 is NOT NULL so i dont need there a NULL check, but dates date2 and date3 can be NULL so i want to check for nulls and accordingly check the date condition. I tried two approaches but cant figure out how to do it right. Can you help?

Query 1:

SELECT *
FROM
    docs
WHERE
    date1 < '20120601'
    AND CASE
        WHEN date2 IS NOT NULL AND date3 IS NOT NULL THEN date2 < '20120601' 
        WHEN date2 IS NOT NULL AND date3 IS NULL THEN date2 < '20120601'
        WHEN date2 IS NULL AND date3 IS NOT NULL THEN date3 < '20120601'
    END

This gives and error in the first when: Incorrect syntax near '<'.

I modified my query to this:

SELECT *
FROM
    docs
WHERE
    date1 < '20120601'
    AND CASE
        WHEN date2 IS NOT NULL AND date3 IS NOT NULL AND date2 < '20120601' THEN TRUE 
        WHEN date2 IS NOT NULL AND date3 IS NULL AND date2 < '20120601' THEN TRUE
        WHEN date2 IS NULL AND date3 IS NOT NULL AND date3 < '20120601' THEN TRUE
    END

and got another error: An expression of non-boolean type specified in a context where a condition is expected, near 'END'.

Answer 1

This should work:

SELECT *
FROM
    docs
WHERE (date1 < '20120601')
OR    (date2 IS NOT NULL AND date2 < '201205601')
OR    (date2 IS NULL AND date3 IS NOT NULL AND date3 < '20120601')

Answer 2

where
date1 < '20120601'
    and
coalesce(date2, date3, '20000101') < '20120601'