Sum(Case when) resulting in multiple rows of the s

I have a huge table of customer orders and I want to run one query to list orders by month for the past 13 months by 'user_id'. What I have now (below) works but instead of only listing one row per user_id it lists one row for each order the user_id has. Ex: one user has 42 total orders over his life with us so it lists his user_id in 42 rows and each row only has one payment. Typically I would just throw this in a pivot table in excel but I'm over the million row limit so I need for it to be right and have had zero success. I would like for the read out to look like this:

user_id | jul_12 | aug_12 |

123456 | 150.00 | 150.00 |

Not this:

user_id | jul_12 | aug_12 |

123456 | 0.00 | 150.00 |

123456 | 150.00 | 0.00 |

etc. 40 more rows

SELECT ui.user_id, 
SUM(CASE WHEN date_part('year', o.time_stamp) = 2012 AND date_part('month', o.time_stamp) = 07 THEN o.amount ELSE 0 END) jul_12,
SUM(CASE WHEN date_part('year', o.time_stamp) = 2012 AND date_part('month', o.time_stamp) = 08 THEN o.amount ELSE 0 END) aug_12,
FROM orders o JOIN users_info ui ON ui.user_id = o.user_id
WHERE user_id = '123456'
GROUP BY ui.user_id, o.time_stamp;

Try something like:

SELECT ui.user_id, 
SUM(CASE WHEN date_part('year', o.time_stamp) = 2012 AND date_part('month', o.time_stamp) = 07 THEN o.amount ELSE 0 END) jul_12,
SUM(CASE WHEN date_part('year', o.time_stamp) = 2012 AND date_part('month', o.time_stamp) = 08 THEN o.amount ELSE 0 END) aug_12,
FROM orders o JOIN users_info ui ON ui.user_id = o.user_id
WHERE user_id = '123456'
GROUP BY ui.user_id;

You were getting one row per order because you were grouping by o.time_stamp and timestamps are different for each order.

A shorter version of query:

SELECT ui.user_id, 
SUM(CASE WHEN date_trunc('month', o.time_stamp) = to_date('2012 07','YYYY MM') THEN o.amount END) jul_12,
SUM(CASE WHEN date_trunc('month', o.time_stamp) = to_date('2012 08','YYYY MM') THEN o.amount END) aug_12,
FROM orders o 
JOIN users_info ui ON ui.user_id = o.user_id
WHERE ui.user_id = '123456'
GROUP BY ui.user_id;