I have one data extension that is about 9 million records (IGO 3 months simple). It has 2 columns, SKU and CustomerID. Both of the fields are NULL so there is duplicated CustomerID's and SKU's.
I'm trying to left join another two data extensions.
First one (Unique ID Repository with Customer ID as PK) that is only about 3 million records.
Second one(Product Name) just over 2 million, where ProductDescriptionID is the PK and ProductName Nullable.
If I run the query with only the first join, I get the 9 million records back. When I join the second one, I get over 40 million records and I don't understand how is this possible when the DE that I'm joining only has just over 2 million records?
SELECT igo.CustomerID, igo.sku, ui.ProductDescriptionID, pn.ProductName from [IGO 3 months simple] igo left join [Unique ID Repository] ui on igo.CustomerID = ui.CustomerID left join [Product Name] pn on ui.ProductDescriptionID = pn.ProductDescriptionID
Can someone explain me how is this possible please? If you need more info please let me know.