SQL：如何从3个表中构建复杂的查询

Question

I have 3 tables with data and I need to select sum of sales (SalesAmount) per country(CountryRegionName) in 2014 fiscal year (FiscalYear).

The first table is Geography .

There are data with CountryRegionName and also SalesTerritoryKey (number which means some districts of CountryRegionName ).

The second table, anong others, has information about OrderDateKey , SalesTerritoryKey and SalesAmount . The OrderDateKey field include records in the following format: "20130828".

The third table, among others, has fields DateKey (in the same format as OrderDateKey from second table and FiscalYear (format like: "2012").

I dont't understand how I can select needed data using all 3 tables because second table has information about year ( OrderDateKey ) in different format which third table supports in field FiscalYear .

Currently I stopped which following statement:

`SELECT CountryRegionName, SUM(SalesAmount) FROM FirstTable  GROUP BY SalesTerritoryKey`

I know that we need to join all tables and make some subqueries but don't understand how to compare dates and countries which are in different formats. Thanks for your help!

Answer 1

SELECT t1.CountryRegionName, SUM(t2.SalesAmount) 
FROM ThirdTable  t3
LEFT JOIN SecondTable t2
ON t3.DateKey = t2.OrderDateKey
LEFT JOIN FirstTable t1
ON t1.SalesTerritoryKey = t2.SalesTerritoryKey
WHERE t3.FiscalYear = "2014"
GROUP BY t1.CountryRegionName