sql basic queries - sql

Question

Im new to sql and playing around with some example databases I have found.
I have a table called student as follows
id name expenditure item
1 dan 45 social
1 dan 60 books
2 sarah 32 food
3 matt 64 food
3 matt 71 social
I was trying to find students who spent money on both food and social but spent more money on social than food.
I tried:
Select name
from student
where item = 'social' and item = 'food'

Answer 1

No individual line of data has item = 'social' AND item = 'food'. As the WHERE clause applies to one row of data at a time, you can't do that.
What you need to do is group all the records together by student, and then check some characteristics of the data in that group.
SELECT
id,
name
FROM
student
GROUP BY
id,
name
HAVING
SUM(CASE WHEN item = 'social' THEN 1 ELSE 0 END) > 0
AND SUM(CASE WHEN item = 'food' THEN 1 ELSE 0 END) > 0
That gives all students who have some records for 'social' and some records for 'food'.
Then, you can add the following to get only those who spent more on 'social' than on 'food'.
AND SUM(CASE WHEN item = 'social' THEN expenditure ELSE 0 END)
>
SUM(CASE WHEN item = 'food' THEN expenditure ELSE 0 END)

Answer 2

where  item = 'social' and item = 'food'  Doesn't work because a single row can never be both. When comparing two rows from the same table the easiest way is to do a self join
SELECT
s1.Name
FROM student s1
INNER JOIN student s2
ON s1.id = s2.id
WHERE
s1.item = 'social' and s2.item = 'food'
and
s1.expenditure > s2.expenditure
Notes:
If you have multiple rows for the same type of expenditures you'll want to use Dem's approach of summing the different types and then comparing
If you want to include people who haven't spent any money on food you'll have to use a LEFT JOIN. You'll also to move the test for 'food' into the ON clause otherwise it will act like an inner join.
LEFT JOIN student s2
ON s1.id = s2.id
and s2.item = 'food'
SQL Fiddle Demo