我有一个查询,我计算多年的分组。有些年份没有可计算的项目,因此没有结果。我想让SPARQL在这些年里返回零。
我正在使用维基数据查询服务https://query.wikidata.org,而我目前的解决方案是制作一系列值并与实际查询结合使用。它对我来说看起来有点笨拙。还有更好的方法吗?
#defaultView:BarChart
select ?year ?number_of_pages ?work_label where {
{
select ?year (sample(?pages) as ?number_of_pages) ?work_label
where {
{
select * where {
values (?year ?pages ?work_label) {
("2000" "0"^^xsd:integer "_")
("2001" "0"^^xsd:integer "_")
("2002" "0"^^xsd:integer "_")
("2003" "0"^^xsd:integer "_")
("2004" "0"^^xsd:integer "_")
("2005" "0"^^xsd:integer "_")
("2006" "0"^^xsd:integer "_")
("2007" "0"^^xsd:integer "_")
("2008" "0"^^xsd:integer "_")
("2009" "0"^^xsd:integer "_")
("2010" "0"^^xsd:integer "_")
("2011" "0"^^xsd:integer "_")
("2012" "0"^^xsd:integer "_")
("2013" "0"^^xsd:integer "_")
("2014" "0"^^xsd:integer "_")
("2015" "0"^^xsd:integer "_")
("2016" "0"^^xsd:integer "_")
}
}
}
union {
?work wdt:P50 wd:Q18921408 .
?work wdt:P1104 ?pages .
?work wdt:P577 ?date .
?work rdfs:label ?long_work_label . filter(lang(?long_work_label) = 'en')
bind(substr(?long_work_label, 1, 20) as ?work_label)
bind(str(year(?date)) as ?year)
}
}
group by ?year ?work ?work_label
order by ?year
}
}
答案 0 :(得分:3)
您不需要这么多嵌套查询。这是一个简单的解决方案:
#defaultView:BarChart
select ?year ?pages ?label
where {
# iterating over years
values ?year {
"2000" "2001" "2002" "2003" "2004" "2005"
"2006" "2007" "2008" "2009" "2010" "2011"
"2012" "2013" "2014" "2015" "2016"
}
# binding defaults
bind( 0 as ?default_pages)
bind("_" as ?default_label)
# if there is a work in the given year, ?work_pages and ?work_label will be bound
optional {
?work wdt:P50 wd:Q18921408;
wdt:P1104 ?work_pages;
wdt:P577 ?work_date.
bind(str(year(?work_date)) as ?year).
?work rdfs:label ?long_work_label.
filter(lang(?long_work_label) = 'en').
bind(substr(?long_work_label, 1, 20) as ?work_label)
}
# either take ?work_pages/label value or default and bind it as the result ?pages/label
bind(coalesce(?work_pages, ?default_pages) as ?pages)
bind(coalesce(?work_label, ?default_label) as ?label)
}
order by ?year
以下是结果截图:
此处的关键是optional
+ bind
/ coalesce
的组合。一般模式是
bind(... as ?default_foo)
optional {
# try to get value ?foo
}
bind(coalesce(?foo, ?default_foo) as ?result_foo)
coalesce
返回它可以的第一个值(绑定/计算没有错误)。因此,如果您尝试在optional { ... }
中获取的值未绑定,则将采用默认值并将其绑定为结果。写一个更冗长的方式:
bind(if(bound(?foo), ?foo, ?default_foo) as ?result_foo)
但coalesce
更好,因为您可以在其中传递多个值。在更复杂的查询中,它可能很有用:请参阅this example。