How to code subqueries

Exercise 1

Write a SELECT statement that returns the same result set as this SELECT statement, but don’t use a join.

Instead, use a subquery in a WHERE clause that uses the IN keyword.

SELECT DISTINCT vendor_name
FROM vendors JOIN invoices
ON vendors.vendor_id = invoices.vendor_id ORDER BY vendor_name

Evercise 2

Write a SELECT statement that answers this question:

Which invoices have a payment total that’s greater than the average payment total for all invoices with a payment total greater than 0?

Return the invoice_number and invoice_total columns for each invoice.

This should return 20 rows.

Sort the results by the invoice_total column in descending order.

Evercise 3

Write a SELECT statement that returns two columns from the General_Ledger_Accounts table: account_number and account_description.

Return one row for each account number that has never been assigned to any line item in the Invoice_Line_Items table.

To do that, use a subquery introduced with the NOT EXISTS operator. This should return 54 rows.

Sort the results by the account_number column.

Evercise 4

Write a SELECT statement that returns four columns: vendor_name, invoice_id, invoice_sequence, and line_item_amount.

Return a row for each line item of each invoice that has more than one line item in the Invoice_Line_Items table.

Hint: Use a subquery that tests for invoice_sequence > 1.

This should return 6 rows. Sort the results by the vendor_name, invoice_id, and invoice_sequence columns.

Ecercise 5

Write a SELECT statement that returns two columns: vendor_id and the largest unpaid invoice for each vendor.

To do this, you can group the result set by the vendor_id column. This should return 7 rows.

Write a second SELECT statement that uses the first SELECT statement in its FROM clause.

The main query should return a single value that represents the sum of the largest unpaid invoices for each vendor.

Exercise 6

Write a SELECT statement that returns the name, city, and state of each vendor that’s located in a unique city and state.

In other words, don’t include vendors that have a city and state in common with another vendor.

This should return 38 rows.

Sort the results by the vendor_state and vendor_city columns.

Exercise 7

Use a correlated subquery to return one row per vendor, representing the vendor’s oldest invoice (the one with the earliest date).

Each row should include these four columns: vendor_name, invoice_number, invoice_date, and invoice_total.

This should return 34 rows.

Sort the results by the vendor_name column.

Exercise 8

Rewrite exercise 7 so it gets the same result but uses an inline view instead of a correlated subquery.