Cross join ignores where clause

Table: Shopping

shop_id shop_building shop_person  shop_time
1   1   Brian   40
2   2   Brian   31
3   1   Tom    20
4   3   Brian   30

Table: buildings

building_id buil开发者_如何学Cding_city
1     London
2     Newcastle 
3     London
4     London

Table:bananas

banana_id  banana_building banana_amount  banana_person
1      2     1      Brian
2      3     1       Brian
2      1     1      Tom

I now want it show me the amount of bananas each person has bought in London.

I used this code:

SELECT tt.*, tu.*, tz.*,
           SUM(shop_time)           AS shoptime, 
           Ifnull(banana_amount, 0) AS bananas 
    INNER JOIN buildings tu ON tt.shop_building=tu.building_id
    FROM   shopping tt 
           LEFT OUTER JOIN (SELECT banana_person, banana_building,
                                   SUM(banana_amount) AS banana_amount 
                            FROM   bananas 
                            GROUP  BY banana_person) tz 
             ON tt.shop_person = tz.banana_person AND tt.shop_building = tz.banana_building
 WHERE tu.building_city = 'London'
    GROUP  BY shop_person; 

But it doesn't work. It's as if I'm telling it too late, that it should only look in London, as it ignores this.

Try this way:

SELECT 
    s.shop_person, sum(b.banana_amount) as Amt, , sum(shop_time) as TimeAmt
FROM bananas b
    INNER JOIN buildings bu ON b.banana_building = bu.building_id
    INNER JOIN shopping s ON bu.building_id = s.shop_building
WHERE
    bu.building_city = N'London'
GROUP BY s.shop_person

This query is different, but it does what you want - 'the amount of bananas each person has bought in London'

Without knowing what database you are using, here is a working version for mssql. I actually rebuilt the tables to make sure it is correct.

For other database systems you'll probably have to use an other function than ISNULL in the SELECT statement.

SELECT tt.shop_person,
    tt.shop_building,
    SUM(tt.shop_time) AS shoptime,
    ISNULL(SUM(tz.banana_amount), 0) AS bananas
FROM dbo.shopping tt
INNER JOIN dbo.buildings tu ON tt.shop_building = tu.building_id
LEFT OUTER JOIN
    (SELECT banana_person, banana_building, SUM(banana_amount) AS banana_amount
        FROM bananas
        GROUP BY banana_person, banana_building) tz
    ON tt.shop_person = tz.banana_person AND tt.shop_building = tz.banana_building
WHERE (tu.building_city = 'London')
GROUP BY tt.shop_person, tt.shop_building

I had to add an aggregate function around tz.banana_amount - which one (SUM, MIN, MAX) doesn't matter.

Result:

shop_person shop_building shoptime bananas
Brian       1             40       0
Tom         1             20       1
Brian       3             30       1

I played around with different amounts in bananas etc. and it works correctly.