PostgreSQL使用交叉表进行一对多的转移

Question

I am trying to build a one to many query in PostgreSQL that looks at a customer table, a transaction table and the result is a table showing which products the customer has purchased.

table: customers
id
1
2
3
4
5

table: purchases
custid  product
1       software 1
1       software 2
3       software 2
3       software 3  
4       software 1
5       software 1
5       software 2
5       software 3

The result I want to get is:

custid  software1   software2   software3
1       TRUE        TRUE        NULL
3       NULL        TRUE        TRUE
4       TRUE        NULL        NULL
5       TRUE        TRUE        TRUE

From what I can tell I need to use crosstab (since postgreSQL doesn't support pivot ), but I'm not sure on the snytax. I'd also be fine if the output included row:

custid  software1   software2   software3
2       NULL        NULL        NULL

If its easier to do it one way or the other, it doesn't really matter.

Answer 1

Ignoring customers that did not buy anything, because that's a bit shorter and faster:

SELECT * FROM crosstab(
    'SELECT custid, product, true FROM purchases ORDER BY 1, 2'    
  , $$VALUES ('software 1'::text), ('software 2'), ('software 3')$$)
AS ct (custid int, software1 bool, software2 bool, software3 bool);

Details:

• PostgreSQL Crosstab Query

One minor difficulty here: you need to add the boolean value in the query, since it's not in the table.

Answer 2

I'm not familiar with crosstab but you can it with a group by clause and a case expression like this(only if there are only those 3 softwares, if its unlimited that this solution is no good):

SELECT t.id,
       max(case when s.product = 'software 1' then true end) as software1,
       max(case when s.product = 'software 2' then true end) as software2,
       max(case when s.product = 'software 3' then true end) as software3
FROM customers t
LEFT OUTER JOIN purchases s ON(t.id = s.custid)
GROUP BY t.id

This will also include a row for the missing ID's.