Using join to match table rows based on a date falling within a range

Question

I am trying to map the World Bank country income levels to a dataset. The income levels change year to year and I need to map the appropriate income level to a transaction using the transaction's country and date.

The following tables illustrate the layout and problem:

Example - Table 1                  Table 2
-----------------                  ----------------------------------------
|ISOCountryCode3, PODate       |   |CountryAlpha3, StartDate,  EndDate,    IncomeClass|
|CIV,            '2009-11-01'  |   | CIV,          1989-10-02, 1989-09-12,     lower  |
|ALB,            '2007-01-04'  |   | CIV,          2009-01-01, 2010-01-01,     lower  |  
                                   | CIV,          2010-01-02, 2011-01-01,     middle |

The system should return:

|CIV, '2009-11-01', lower|
|...

Here's the SQL that I've tried so far

SELECT mergestandard.*,incomeclassifications.IncomeClass 
FROM `mergestandard`
LEFT OUTER JOIN  incomeclassifications 
ON mergestandard.ISOCountryCode3 = incomeclassifications.CountryAlpha3
AND mergestandard.PODate<=incomeclassifications.EndDate
AND mergestandard.PODate>=incomeclassifications.StartDate

Unfortunately, the system keeps returning null in the incomeclass field. Any help would be much appreciated!

Answer 1

I don't usually use mySql, but I would try this:

SELECT mergestandard.*,incomeclassifications.IncomeClass 
FROM `mergestandard`
LEFT OUTER JOIN  incomeclassifications 
ON mergestandard.ISOCountryCode3 = incomeclassifications.CountryAlpha3
WHERE
mergestandard.PODate<=incomeclassifications.EndDate
AND mergestandard.PODate>=incomeclassifications.StartDate

The first part would give you

|CIV, '2009-11-01', lower|
|CIV, '2009-11-01', lower|
|CIV, '2009-11-01', middle|
|...

Which actually also includes the dates

|CIV, '2009-11-01', lower| 1989-10-02, 1989-09-12
|CIV, '2009-11-01', lower| 2009-01-01, 2010-01-01
|CIV, '2009-11-01', middle| 2010-01-02, 2011-01-01
|...

Then use the where clause to filter what you want.

|CIV, '2009-11-01', lower| 2009-01-01, 2010-01-01