I've looked all over and haven't yet found an intelligent way to handle this, though I feel sure one is possible:

One table of historical data has quarterly information:

CREATE TABLE Quarterly (
unique_ID INT UNSIGNED NOT NULL,
date_posted DATE NOT NULL,
datasource TINYINT UNSIGNED NOT NULL,
data FLOAT NOT NULL,
PRIMARY KEY (unique_ID));

Another table of historical data (which is very large) contains daily information:

CREATE TABLE Daily (
unique_ID INT UNSIGNED NOT NULL,
date_posted DATE NOT NULL,
datasource TINYINT UNSIGNED NOT NULL,
data FLOAT NOT NULL,
qtr_ID INT UNSIGNED,
PRIMARY KEY (unique_ID));

The qtr_ID field is not part of the feed of daily data that populated the database - instead, I need to retroactively populate the qtr_ID field in the Daily table with the Quarterly.unique_ID row ID, using what would have been the most recent quarterly data on that Daily.date_posted for that data source.

For example, if the quarterly data is

101 2009-03-31 1 4.5
102 2009-06-30 1 4.4
103 2009-03-31 2 7.6
104 2009-06-30 2 7.7
105 2009-09-30 1 4.7

and the daily data is

1001 2009-07-14 1 3.5 ??
1002 2009-07-15 1 3.4 &&
1003 2009-07-14 2 2.3 ^^

then we would want the ?? qtr_ID field to be assigned '102' as the most recent quarter for that data source on that date, and && would also be '102', and ^^ would be '104'.

The challenges include that both tables (particularly the daily table) are actually very large, they can't be normalized to get rid of the repetitive dates or otherwise optimized, and for certain daily entries there is no preceding quarterly entry.

I have tried a variety of joins, using datediff (where the challenge is finding the minimum value of datediff greater than zero), and other attempts but nothing is working for me - usually my syntax is breaking somewhere. Any ideas welcome - I'll execute any basic ideas or concepts and report back.