Joining to landing pages query doubles the sessions per source - google-analytics

Question

I'm trying to query sum of visits per source from a Big Query table of Google Analytics data, but will need to filter some sessions out at landing page level. Hence I'm pre-querying visitIDs by landing page and re-joining to session data like so:
#StandardSQL
WITH landingpages AS (
SELECT
visitID,
h.page.pagePath AS LandingPage
FROM
`project.dataset.ga_sessions_*`, UNNEST(hits) AS h
WHERE
hitNumber = 1
AND
_TABLE_SUFFIX BETWEEN '20150926' AND '20150926'
# filters to be added here
)
SELECT
sessions.trafficSource.source,
SUM(sessions.totals.visits) AS visits
FROM `project.dataset.ga_sessions_*` AS sessions
JOIN
landingpages
ON
landingpages.visitID = sessions.visitID
WHERE
_TABLE_SUFFIX BETWEEN '20150926' AND '20150926'
GROUP BY
trafficSource.source
ORDER BY
visits DESC
This roughly doubles the number of sessions per each source as reported from GA.
Can anyone point out what I've done wrong? (I suspect it is blindingly obvious)
I've tried examining the data output from the first query and can't find anything wrong with it aside from a very small proportion of duplicated visitIDs. I've also tried various different types of JOIN, all to now avail.

Answer 1

When querying ga data from GBQ it's imperative to know and keep in mind that a unique visit is represented by both a fullVisitorID and visitID. Only a double join on both will return a meaningful data set.
Here's what I should have written:
#StandardSQL
WITH landingpages AS (
SELECT
fullVisitorId,
visitID,
h.page.pagePath AS LandingPage
FROM
`project.dataset.ga_sessions_*`, UNNEST(hits) AS h
WHERE
hitNumber = 1
AND
_TABLE_SUFFIX BETWEEN '20150926' AND '20150926'
),
session_data AS (
SELECT
date AS ga_date, trafficSource.source AS source, fullVisitorId, visitID, SUM(totals.visits) AS visits
FROM
`project.dataset.ga_sessions_*`
WHERE
_TABLE_SUFFIX BETWEEN '20150926' AND '20150926'
AND
totals.visits > 0
GROUP BY ga_date, source, fullVisitorId, visitID
)
SELECT
ga_date, source, SUM(visits) AS Sessions
FROM
landingpages
JOIN
session_data
ON
landingpages.VisitID = session_data.VisitID
AND
landingpages.fullVisitorId = session_data.fullVisitorId
GROUP BY
ga_date, source
ORDER BY
Sessions DESC