added exercise solutions

exercise-01.sql

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+SELECT vendor_id, SUM(invoice_total) AS invoice_total_sum
+FROM invoices
+GROUP BY vendor_id

exercise-02.sql

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+SELECT vendor_name, SUM(payment_total) AS payment_total_sum
+FROM vendors v JOIN invoices i
+  ON v.vendor_id = i.vendor_id
+GROUP BY vendor_name
+ORDER BY payment_total_sum DESC

exercise-03.sql

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+SELECT vendor_name, COUNT(*) AS invoice_count,
+       SUM(invoice_total) AS invoice_total_sum
+FROM vendors v JOIN invoices i
+  ON v.vendor_id = i.vendor_id
+GROUP BY vendor_name
+ORDER BY invoice_count DESC

exercise-04.sql

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+SELECT account_description, COUNT(*) AS line_item_count,
+       SUM(line_item_amount) AS line_item_amount_sum
+FROM general_ledger_accounts gl 
+  JOIN invoice_line_items li
+    ON gl.account_number = li.account_number
+GROUP BY account_description
+HAVING line_item_count > 1
+ORDER BY line_item_amount_sum DESC

exercise-05.sql

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+SELECT account_description, COUNT(*) AS line_item_count,
+       SUM(line_item_amount) AS line_item_amount_sum
+FROM general_ledger_accounts gl 
+  JOIN invoice_line_items li
+    ON gl.account_number = li.account_number
+  JOIN invoices i
+    ON li.invoice_id = i.invoice_id
+WHERE invoice_date BETWEEN '2018-04-01' AND '2018-06-30'
+GROUP BY account_description
+HAVING line_item_count > 1
+ORDER BY line_item_amount_sum DESC

exercise-06.sql

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+SELECT account_number, SUM(line_item_amount) AS line_item_sum
+FROM invoice_line_items
+GROUP BY account_number WITH ROLLUP

exercise-07.sql

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+SELECT vendor_name,
+       COUNT(DISTINCT li.account_number) AS number_of_gl_accounts
+FROM vendors v 
+  JOIN invoices i
+    ON v.vendor_id = i.vendor_id
+  JOIN invoice_line_items li
+    ON i.invoice_id = li.invoice_id
+GROUP BY vendor_name
+HAVING number_of_gl_accounts > 1
+ORDER BY vendor_name