SQL-complete-tutorial

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USE sql_invoicing;
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SELECT
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    invoice_id,
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    invoice_total,
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    (SELECT AVG(invoice_total)
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	FROM invoices) AS invoice_average,
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	invoice_total - (SELECT invoice_average) AS difference
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FROM invoices;
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-- show all clients with thir total_sales for issued invoices
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SELECT 
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    client_id,
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    name,
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    (SELECT SUM(invoice_total)
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	FROM invoices
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        WHERE client_id = c.client_id) AS total_sales,
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    (SELECT AVG(invoice_total) 
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	FROM invoices) AS average, 
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    (SELECT total_sales - average) AS difference
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FROM clients c;
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