Filling in for missing latest data with last available data - sql

Question

I have two tables, one (market_cap_data) with month_end_date, id, market_cap fields:
month_end_date id market_cap
2012-12-31 123456 5000
2011-12-31 123456 4000
and a second table (start_date_table) with month_end_date, id, start_date fields:
month_end_date id start_date
2011-12-31 123456 1980-12-31
I want to combine the two tables but the start_date_table data ends a year before the market_cap_data table. I want to fill the latest data where the start_date_table doesn't have data using the most recent start_date. For example, instead of an outside join like:
month_end_date id market_cap start_date
2012-12-31 123456 5000 NULL
2011-12-31 123456 4000 1980-12-31
I want it to look like
month_end_date id market_cap start_date
2012-12-31 123456 5000 1980-12-31
2011-12-31 123456 4000 1980-12-31
Tried a bunch of different things but can't figure it out.
Any help would be appreciated!

Answer 1

SELECT
m.month_end_date,
m.id,
m.market_cap,
CASE
WHEN s.start_date IS NOT NULL THEN s.start_date
ELSE (SELECT MAX(s2.start_date) FROM start_date_table s2 WHERE s2.id = m.id)
END AS start_date
FROM market_cap_data m
LEFT JOIN start_date_table s
ON m.id = s.id
AND m.month_end_date = s.month_end_date

Answer 2

I think you would benefit from a case statement, this is not tested as I don't have a fiddle to validate against
create function get_latest_date_from_table(varchar(100) table_name returns Date
(
return select max(date) from #table_name
)
create procedure modify_null_dates_for_marker
(
max_date Date;
max_date = get_latest_date_from_table('table');
select
foo,
bar
CASE WHEN start_date IS NULL
THEN max_date
ELSE start_date END AS start_date
FROM table
)
This should give a method to set the null columns correctly.