Add 2024-02-07 stuff

notes/database-prog/sql-scripts/2024-02-05.sql

@@ -59,7 +59,7 @@ USE AP
 -- it doesn't matter which column, so * is fine
 SELECT COUNT(*)
 FROM Invoices
-WHERE VendorId = 123
+WHERE VendorID = 123
 
 -- How many invoices in 2019
 SELECT COUNT(*) TotalInvoices
@@ -76,3 +76,51 @@ WHERE YEAR(InvoiceDueDate) = 2020
 -- sum, average, min, and max invoice totals
 SELECT SUM(InvoiceTotal) SumOfInvoices, AVG(InvoiceTotal) AvgInvoice, MIN(InvoiceTotal) SmallestInvoice, MAX(InvoiceTotal) LargestInvoice
 FROM Invoices
+
+----------------
+-- 2024-02-07 --
+----------------
+
+-- Get vendors with 3 or more invoices
+SELECT i.VendorID, v.VendorName, COUNT(*)
+FROM Invoices i
+    JOIN Vendors v ON i.VendorID = v.VendorID
+GROUP BY i.VendorID, v.VendorName
+HAVING COUNT(*) >= 3
+ORDER BY i.VendorID
+
+-- CHAPTER 6 --
+
+-- Get vendors with no invoices
+SELECT VendorID
+FROM Vendors
+WHERE VendorID NOT IN (SELECT VendorID
+FROM Invoices)
+ORDER BY VendorID
+
+-- Sub-queries aren't ideal, they perform worse than JOINs. With a JOIN:
+SELECT v.VendorID
+FROM Vendors v
+    LEFT JOIN Invoices i ON v.VendorID = i.VendorID
+WHERE InvoiceID IS NULL
+
+
+-- Get all invoices > average invoice total
+-- Usually can't get rid of the sub-query for aggregate stuff like this
+SELECT InvoiceID
+FROM Invoices
+WHERE InvoiceTotal > (SELECT AVG(InvoiceTotal)
+FROM Invoices)
+ORDER BY InvoiceTotal
+
+-- Get all invoices > average invoice total by vendor
+-- It's got a sub-query, but it only runs once so the performance isn't *that* bad
+SELECT TOP 10 InvoiceID, InvoiceTotal, i.VendorID, AvgTotal
+FROM Invoices i
+    JOIN (SELECT VendorID, AVG(InvoiceTotal) AvgTotal
+    FROM Invoices
+    GROUP BY VendorID) AvgTotal ON i.VendorID = AvgTotal.VendorID
+WHERE InvoiceTotal >
+(SELECT AVG(InvoiceTotal)
+FROM Invoices)
+ORDER BY i.VendorID, InvoiceTotal