MySQL：子查询的麻烦

Question

So I am attempting my very first sub query and ran into a small problem... Why is the query not taking into account my WHERE clause?

My query:

SELECT *
FROM Product
WHERE stdUnitPrice < 5* 
(SELECT MIN(stdUnitPrice)
FROM Product
WHERE discontinued = 'A')

But in the results I am still getting values in the discontinued column that are NOT just 'A'

Here are the results:

# productId, prodName, stdUnitPrice, qtyPerUnit, discontinued
3, Aniseed Syrup, 11.00, 12 - 550 ml bottles, A
13, Konbu, 6.60, 2 kg box, A
19, Teatime Chocolate Biscuits, 10.12, 10 boxes x 12 pieces, A
21, Sir Rodney's Scones, 11.00, 24 pkgs. x 4 pieces, A
23, Tunnbrod, 9.90, 12 - 250 g pkgs., A
**24, Guarana Fantastica, 4.95, 12 - 355 ml cans, D**
33, Geitost, 2.75, 500 g, A
41, Jack's New England Clam Chowder, 10.61, 12 - 12 oz cans, A
45, Rogede sild, 10.45, 1k pkg., A
46, Spegesild, 13.20, 4 - 450 g glasses, A
47, Zaanse koeken, 10.45, 10 - 4 oz boxes, A
52, Filo Mix, 7.70, 16 - 2 kg boxes, A
54, Tourtiere, 8.19, 16 pies, A
74, Longlife Tofu, 11.00, 5 kg pkg., A
75, Rhonbrau Klosterbier, 8.52, 24 - 0.5 l bottles, A
78, Bob's Down Home Juice, 7.90, 6 pack, A

Answer 1

Try this:

select *
from Product
where stdUnitPrice < (5 * (select min(stdUnitPrice) from Product ) )
 and discontinued = 'A'

Answer 2

It appears as though MySQL is looking at the where and thinking stdUnitPrice < 5

As well, your where needs to be on the main query.

Try moving your math into the subquery like so:

SELECT *
FROM Product
WHERE stdUnitPrice <
(SELECT 5*MIN(stdUnitPrice)
FROM Product)
    AND discontinued = 'A'

Answer 3

So the reasoning for this not working the way you want it to is because you are missing the where clause in your first query. Let me break it down a little for you.

select * from Product where stdUnitPrice < 5* (select min(stdUnitPrice)   from Product where discontinued = 'A')

so in this query the sub query gets executed first so lets assume that we set the sub query to a variable:

var subquery = select min(stdUnitPrice)   from Product where discontinued = 'A';

Now after that subquery is executed it is then plugged back into the original query like so: (using the new variable name)

select * from Product where stdUnitPrice < 5 * (subquery);

so to get this to include a where discontinued = 'A' you would need to change your query to the following:

SELECT * FROM Product WHERE stdUnitPrice < 5 * (SELECT MIN(stdUnitPrice) FROM Product WHERE discontinued = 'A') AND discontinued = 'A';

I hope this helps.

EDIT: Just to clarify you can't actually use a variable like that im just using that as an example to show how the query will actually be executed.

Answer 4

You are performing an aggregate subset operation that is filtered, however, your main query will return all rows. You are basically asking to return all rows * the minimum value in the table. You need t return the minimum value within all rows that match your selection clause.