Cypher: Correlated variables in Union

Using a UNION:

// Top 3 cities
MATCH (t:Store)<-[:HasStore]-(s:Sales)
RETURN t.StoreCity AS StoreCity, sum(s.StoreSales) AS StoreSales
ORDER BY sum(s.StoreSales) DESC LIMIT 3
// All other cities
UNION
MATCH (t:Store)<-[:HasStore]-(s:Sales)
WITH t.StoreCity, sum(s.StoreSales) AS AllStoreSales
ORDER BY AllStoreSales DESC SKIP 3
RETURN "Other cities" AS StoreCity, sum(AllStoreSales) AS StoreSales

The UNION runs two distinct queries, so before the UNION we find the top three selling cities and return their names and counts, as you had originally

After the UNION, the same basic query is run again to find the sum of sales by city, ordered by descending sales, then the top three results are skipped. The remaining city's sales are summed and returned as Other cities.

The two parts of the UNION are completely separate, but both queries have to return the same number of columns with the same column names and in the same column order.