SQL Left join Query Issue - sql

Question

I have the following tables
Users
ID Name
1 John Smith
2 James Jones
3 Peter Brown
Purchases
USERID NAME
1 Apple
1 Pear
1 Banana
2 Apple
2 Pear
3 Apple
How can i have a sql query that returns users that brought (apple & pear & banana)
So in the above tables it would return only 'John Smith'
Thanks

Answer 1

This is an example of a set-within-sets query. A flexible way to solve this is using group by and having:
select userid
from purchases
where name in ('apple', 'pear', 'banana')
group by userid
having count(*) = 3;
To get the name, you would join in the Users table.
If, unlike in your sample data, duplicates are allowed in the table, then use count(distinct name) = 3 in the having clause.

Answer 2

You can JOIN both tables and COUNT products:
SELECT u.Name
FROM Purchases p
JOIN Users u
ON p.UserID = u.ID
WHERE p.Name IN ('Apple', 'Pear', 'Banana')
GROUP BY userid
HAVING COUNT(DISTINCT p.Name) = 3;
SqlFiddleDemo
There is possibility that user has bought 2 apples and one banana so you should count distinct product names.
If you are using Postgresql/SQL Server/Oracle you need to wrap Name with aggregation function:
SELECT MAX(u.Name) AS Name
FROM Purchases p
JOIN Users u
ON p.UserID = u.ID
WHERE p.Name IN ('Apple', 'Pear', 'Banana')
GROUP BY userid
HAVING COUNT(DISTINCT p.Name) = 3;

Answer 3

SQL Fiddle
SELECT users.name, purchased.items
FROM users
INNER JOIN (
select userid as id, array_agg(distinct name) items
from purchases
group by userid
having array_length(array_agg(distinct name),1)=3
) purchased USING(id);
This does not work if you are only looking for those specific items. If there are other items that could be purchased and you are only looking for users that purchased those specific items, you would have to do something similar to what Gordon, with a correction for the number of times an item was purchased.