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Orders above a VAT threshold · SQL scenario · D8LooP

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Orders above a VAT threshold

VAT is 20% of the order total. Accounting wants the orders where it comes to more than 20.00, with the VAT shown alongside. Return one row per qualifying order, largest VAT first, ordered by order_id when two are equal.

Result columns · in this order

`order_id`	The order.
`order_total`	Its total.
`vat`	20% of the total, to 2 decimal places.

How to approach it

Repeat the expression where the alias is not available yet, and use the alias where it is.

Sample input

shop_ordersOne row per online order.

order_id	customer_id	placed_at	status	order_total
1001	c1	2026-01-05 10:00:00	paid	120
1002	c2	2026-01-12 14:30:00	Paid	80
1003	c1	2026-01-20 09:15:00	pending	45
1004	c3	2026-01-31 23:30:00	PAID	200
1005	c2	2026-02-01 00:15:00	paid	60
1006	c4	2026-02-03 11:00:00	paid	150
1007	c3	2026-02-10 16:45:00	refunded	90
1008	c1	2026-02-14 12:00:00	paid	300
1009	c5	2026-02-18 08:30:00	Paid	75
1010	c4	2026-02-25 19:20:00	pending	85
1011	c5	2026-02-28 21:00:00	paid	130
1012	c2	2026-03-02 10:10:00	paid	95

12 rows — scroll inside the table to see them all.

Expected output

The exact result your query should return for this input.

order_id	order_total	vat
1008	300	60
1004	200	40
1006	150	30
1011	130	26
1001	120	24

5 rows — all rows shown.

Constraints

1vat is 20% of order_total, rounded to 2 decimal places.
2The threshold applies to the VAT, not to the order total.
3Five orders qualify.
4Sort by vat descending, then by order_id so the result is reproducible.

Worked example

Order 1008 is worth 300, so its VAT is 60 and it belongs in the result. Writing WHERE vat > 20 to find it fails with no such column: vat. SQL evaluates FROM, then WHERE, then GROUP BY, then HAVING, then SELECT, and finally ORDER BY. The alias is created in SELECT, which has not run when WHERE is evaluated — so the name does not exist yet. ORDER BY runs last, which is why the same alias works perfectly well there.

What this tests

Clause evaluation order, and the practical consequence that a SELECT alias is visible to ORDER BY but not to WHERE.

SQL

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