Add SQL query examples

examples/example_queries.sql

@@ -0,0 +1,29 @@
+SELECT
+    c.CustomerId,
+    c.FirstName,
+    c.LastName,
+    SUM(i.Total) AS TotalSpent
+FROM Customers c
+JOIN Invoices i ON c.CustomerId = i.CustomerId
+GROUP BY c.CustomerId, c.FirstName, c.LastName
+ORDER BY TotalSpent DESC;
+
+
+-- Find albums with more than 10 tracks
+SELECT
+    a.AlbumId,
+    a.Title,
+    COUNT(t.TrackId) AS TrackCount
+FROM Albums a
+JOIN Tracks t ON a.AlbumId = t.AlbumId
+GROUP BY a.AlbumId, a.Title
+HAVING COUNT(t.TrackId) > 10;
+
+-- Customers who never made a purchase
+SELECT
+    c.CustomerId,
+    c.FirstName,
+    c.LastName
+FROM Customers c
+LEFT JOIN Invoices i ON c.CustomerId = i.CustomerId
+WHERE i.InvoiceId IS NULL;