Coalescing rows based on field value in row.

Question

I have a table with tax rates where NULL entity type rows represent the default tax rates.

Year End | EntityType | RateType | TaxRate
------------------------------------------
2009 |       NULL | Interest |      13
2009 |       NULL |    Other |       8
2009 |       NULL | Interest |      13
2010 |       NULL |    Other |       9 
2009 |    Company | Interest |      15
2010 | Individual |    Other |       6  

I want to create a stored procedure for this table with Entitytype as a parameter. Such that for each year, it returns the tax rate for that entity type if it exists, otherwise return the NULL row for that year and ratetype.

i.e.: querying the above with entitytype individual should return

Year End | EntityType | RateType | TaxRate
------------------------------------------
2009     |       NULL | Interest |      13
2009     |       NULL |    Other |       8
2009     |       NULL | Interest |      13
2010     | Individual |    Other |       6

Can anybody suggest a way I could achieve this?

Thanks in advance,

Yong

Answer 1

SELECT 
    t1.year_end,
    t1.entity_type,
    t1.ratetype,
    COALESCE(t2.taxrate, t1.taxrate)
  FROM
    Rates AS t1
LEFT JOIN
    Rates AS t2 ON
        t1.year_end = t2.year_end
        AND t2.entitytype IS NULL
WHERE t1.year_end = @year
    AND t1.entitytype = @entitytype