neo4j/cypher: WITH, COLLECT & EXTRACT
Date: 2013-03-20 02:54:43

As I http://www.markhneedham.com/blog/2013/03/20/neo4jcypher-getting-the-hang-of-the-with-statement/[mentioned in my last post] I'm trying to get the hang of the +++<cite>+++http://docs.neo4j.org/chunked/milestone/query-with.html[WITH]+++</cite>+++ statement in neo4j's http://docs.neo4j.org/chunked/milestone/cypher-query-lang.html[cypher] query language and I found another application when trying to work out which opponents teams played on certain days.

I started out with a query which grouped the data set by day and showed the opponents that were played on that day:

[source,cypher]
----

START team = node:teams('name:"Manchester United"')
MATCH team-[h:home_team|away_team]-game-[:on_day]-day
RETURN DISTINCT day.name, COLLECT(TRIM(REPLACE(REPLACE(game.name, "Manchester United", ""), "vs", "")))
----

[source,text]
----

+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| day.name    | opponents                                                                                                                                                                                                                       |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| "Sunday"    | ["Liverpool","Everton","Southampton","Liverpool","Newcastle United","Chelsea","Manchester City","Swansea City","Tottenham Hotspur"]                                                                                             |
| "Wednesday" | ["Southampton","West Ham United","Newcastle United"]                                                                                                                                                                            |
| "Monday"    | ["Everton"]                                                                                                                                                                                                                     |
| "Saturday"  | ["Reading","Fulham","Wigan Athletic","Tottenham Hotspur","Stoke City","Arsenal","Queens Park Rangers","Sunderland","West Bromwich Albion","Norwich City","Reading","Aston Villa","Norwich City","Fulham","Queens Park Rangers"] |
| "Tuesday"   | ["Wigan Athletic"]                                                                                                                                                                                                              |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
5 rows
----

The way we've got the opponents is a bit of a hack - the name of the two teams is in the 'name' property of a game node and we've removed 'Manchester United' and the word 'vs' to get the opponent's name.

I thought it'd be cool if we could separate the games on each day based on whether Manchester United were playing at home or away.

With https://groups.google.com/forum/?fromgroups=#!topic/neo4j/D4M1gXKwQ3U[a lot of help from Wes Freeman] we ended up with the following query which does the job:

[source,cypher]
----

START team = node:teams('name:"Manchester United"')
MATCH team-[h:home_team|away_team]-game-[:on_day]-day
WITH day.name as d, game, team, h
MATCH team-[:home_team|away_team]-game-[:home_team|away_team]-opp
WITH d, COLLECT([type(h),opp.name]) AS games
RETURN d,
  EXTRACT(c in FILTER(x in games: HEAD(x) = "home_team") : HEAD(TAIL(c))) AS home,
  EXTRACT(c in FILTER(x in games: HEAD(x) = "away_team") : HEAD(TAIL(c))) AS away
----

We use a similar approach with COLLECT as in the http://www.markhneedham.com/blog/2013/03/20/neo4jcypher-getting-the-hang-of-the-with-statement/[previous post] whereby we have a collection of tuples describing whether Manchester United were at home or not and who they were playing.

A neat thing that Wes pointed out is that since there are only 2 teams per game we're able to get the opponent node easily because it's the only other node that can match the 'home_team|away_team" relationship since we've already matched our team.

If we run the query just up to the last WITH we get the following result:

[source,text]
----

+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| d           | games                                                                                                                                                                                                                                                                                                                                                                                                                                             |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| "Sunday"    | [["home_team","Liverpool"],["home_team","Everton"],["away_team","Southampton"],["away_team","Liverpool"],["away_team","Newcastle United"],["away_team","Chelsea"],["away_team","Manchester City"],["away_team","Swansea City"],["away_team","Tottenham Hotspur"]]                                                                                                                                                                                 |
| "Wednesday" | [["home_team","Southampton"],["home_team","West Ham United"],["home_team","Newcastle United"]]                                                                                                                                                                                                                                                                                                                                                    |
| "Monday"    | [["away_team","Everton"]]                                                                                                                                                                                                                                                                                                                                                                                                                         |
| "Saturday"  | [["home_team","Reading"],["home_team","Fulham"],["home_team","Wigan Athletic"],["home_team","Tottenham Hotspur"],["home_team","Stoke City"],["home_team","Arsenal"],["home_team","Queens Park Rangers"],["home_team","Sunderland"],["home_team","West Bromwich Albion"],["home_team","Norwich City"],["away_team","Reading"],["away_team","Aston Villa"],["away_team","Norwich City"],["away_team","Fulham"],["away_team","Queens Park Rangers"]] |
| "Tuesday"   | [["away_team","Wigan Athletic"]]                                                                                                                                                                                                                                                                                                                                                                                                                  |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
5 rows
----

We then use the http://docs.neo4j.org/chunked/milestone/query-function.html#functions-filter[FILTER] function to choose either the opponents Manchester United played at home or away and then we use the http://docs.neo4j.org/chunked/milestone/query-function.html#functions-extract[EXTRACT] function to get the opponent from the tuple:

[source,text]
----

+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| d           | home                                                                                                                                                      |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| "Sunday"    | ["Liverpool","Everton"]                                                                                                                                   |
| "Wednesday" | ["Southampton","West Ham United","Newcastle United"]                                                                                                      |
| "Monday"    | []                                                                                                                                                        |
| "Saturday"  | ["Reading","Fulham","Wigan Athletic","Tottenham Hotspur","Stoke City","Arsenal","Queens Park Rangers","Sunderland","West Bromwich Albion","Norwich City"] |
| "Tuesday"   | []                                                                                                                                                        |
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
5 rows

+-----------------------------------------------------------------------------------------------------------------------------+
| d           | away                                                                                                          |
+-----------------------------------------------------------------------------------------------------------------------------+
| "Sunday"    | ["Southampton","Liverpool","Newcastle United","Chelsea","Manchester City","Swansea City","Tottenham Hotspur"] |
| "Wednesday" | []                                                                                                            |
| "Monday"    | ["Everton"]                                                                                                   |
| "Saturday"  | ["Reading","Aston Villa","Norwich City","Fulham","Queens Park Rangers"]                                       |
| "Tuesday"   | ["Wigan Athletic"]                                                                                            |
+-----------------------------------------------------------------------------------------------------------------------------+
5 rows
----

_(I ran the query twice alternating between the last two lines so that it's readable here. In actual fact the away teams would be in a column next to the home teams)_

I thought it was quite interesting how many games Manchester United play away on a Sunday - I think all of those games were probably televised so I thought they'd be more evenly split between home and away matches. Adding televised matches is perhaps http://www.markhneedham.com/blog/2012/07/21/neo4j-embracing-the-sub-graph/[another layer to add to the graph].

It's probably more useful to summarise how many games were played on each day at home and away rather than who they're against and we can use the +++<cite>+++REDUCE+++</cite>+++ function to do this:

[source,cypher]
----

START team = node:teams('name:"Manchester United"')
MATCH team-[h:home_team|away_team]-game-[:on_day]-day
WITH day.name as dayName, game, team, h
MATCH team-[:home_team|away_team]-game-[:home_team|away_team]-opp
WITH dayName, COLLECT([type(h),opp.name]) AS games
RETURN dayName,
  REDUCE(homeGames=0, game in EXTRACT(c in FILTER(x in games: head(x) = "home_team") : HEAD(TAIL(c))) : homeGames + 1) as home,
  REDUCE(awayGames=0, game in EXTRACT(c in FILTER(x in games: head(x) = "away_team") : HEAD(TAIL(c))) : awayGames + 1) as away,
  REDUCE(totalGames=0, game in games : totalGames + 1) as total
----

[source,text]
----

+-----------------------------------+
| dayName     | home | away | total |
+-----------------------------------+
| "Sunday"    | 2    | 7    | 9     |
| "Wednesday" | 3    | 0    | 3     |
| "Monday"    | 0    | 1    | 1     |
| "Saturday"  | 10   | 5    | 15    |
| "Tuesday"   | 0    | 1    | 1     |
+-----------------------------------+
5 rows
----

An alternative way of writing the initial query would be the following which Michael Hunger suggested on the thread:

[source,cypher]
----

START team = node:teams('name:"Manchester United"')
MATCH p=team-[:home_team|away_team]-game-[:home_team|away_team]-(), game-[:on_day]-day
WITH day.name as dayName, COLLECT([LAST(p), HEAD(RELS(p))]) AS opponents
WITH dayName,
  EXTRACT(y in FILTER(x in opponents: TYPE(HEAD(TAIL(x))) = "home_team") : HEAD(y)) AS home,
  EXTRACT(y in FILTER(x in opponents : TYPE(HEAD(TAIL(x))) = "away_team") : HEAD(y)) AS away
RETURN dayName,
  EXTRACT(team in home: team.name) AS homeOpponents,
  EXTRACT(team in away: team.name) AS awayOpponents
ORDER BY dayName
----

Here we take a slightly different approach where we make use of functions that we can apply to a matching path. We create a collection of tuples where +++<cite>+++LAST(p)+++</cite>+++ matches the opponent node and +++<cite>+++HEAD(RELS(p))+++</cite>+++ matches the 'home_team' or 'away_team' relationship accordingly.

We then filter the collection to find the times that we played at home and away. This is done by taking the second value from the tuple and then calling +++<cite>+++TYPE+++</cite>+++ on it which either returns 'home_team' or 'away_team'. We then extract the first value from the tuple which is the opponent node.

In the last part of the query we extract the name from the opponent nodes.