SPARQL basic introductory exercise

1、 Preface

Knowledge map ：Freebase, The server ：virtuoso

SPARQL Basic grammar table

One SPARQL Examples

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?x
WHERE {
FILTER (?x != ns:m.0160w)
FILTER (!isLiteral(?x) OR lang(?x) = '' OR langMatches(lang(?x), 'en'))
ns:m.0160w ns:location.country.currency_used ?x .
}

Roughly speaking

1. PREFIX ns: <http://rdf.freebase.com/ns/>——PREFIX Declaration abbreviation , hinder ns Express http://rdf.freebase.com/ns/. for example ,ns:m.0160w When executed by the interpreter , Equivalent to http://rdf.freebase.com/ns/m.0160w.
2. SELECT DISTINCT ?x—— Search for variables x（?x）, And the search results require duplicate removal （DISTINCT）
3. WHERE {}—— This clause follows SELECT after ,{} Content represents search SELECT Specific screening criteria for .
4. FILTER (?x != ns:m.0160w)—— Required variable x No ns:m.0160w
5. FILTER (!isLiteral(?x) OR lang(?x) = '' OR langMatches(lang(?x), 'en'))—— This clause is a bit complicated , It is a combination of three clauses , Respectively indicate requirements ：
   1. or , Variable x It's not text （!isLiteral）
   2. or ,x The language of is an empty string
   3. or ,x My language is English （en）
6. ns:m.0160w ns:location.country.currency_used ?x .—— This is actually a A triple . Said variable x There is an edge on , By entity ns:m.0160w Pointed out that , Through relationships ns:location.country.currency_used, Point to variable x. It is worth noting that , Any such triplet clause , It should be followed by an English full stop .（ See red dot ）, Otherwise, the interpreter will report an error .

stay virtuoso After the server executes , Format as html, give the result as follows .

Besides , It is worth noting that , about

SELECT DISTINCT ?x

for , If written as

SELECT DISTINCT ?x1 ?x2 ?x3

Then it returns the following （where In Clause ） Of the constraints (x1, x2, x3) Combine .

2、 Basic operation （ From simple to complex ）

2.1 Unconditional query

The simplest form

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT ?e1 ?r ?e2
where {?e1 ?r ?e2}

This means , Without any restriction , Query all triples in the graph . Naturally, there are many returned results ：

You can see To the top of the returned result , We also wrote our own name in the query statement e1,r,e2.

2.2  The conditional query of a single triplet is known

Add a triple constraint

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?e1 ?r
WHERE {
?e1 ?r ns:m.0160w .
}

I.e. requirement Variable e1 and r, Satisfy triples (?e1 ?r ns:m.0160w). give the result as follows ：

2.3 nested queries

Of course , We can also nest other triples （ nested queries ）, such as

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?e1 ?r
WHERE {
?e1 ?r ns:m.0160w .
?e1 ns:freebase.user_profile.hometown ?e2 .
}

obviously , Here we add two triple constraints . The number of results is naturally much less .

2.4 Filter parameters FILTER

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?e1 ?r
WHERE {
?e1 ?r ns:m.0160w .
FILTER(?r=ns:freebase.user_profile.hometown)
}

Filter out （ namely , as long as This part ） Variable r yes ns:freebase.user_profile.hometown The situation of . give the result as follows ：

2.5  Optional constraint OPTIONAL

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?e1 ?r ?e2
WHERE {
?e1 ?r ns:m.0160w .
OPTIONAL { ?e1 ns:freebase.user_profile.hometown ?e2 }
}

such Words , The second triplet constraint is optional , If the conditions （?e1,ns:freebase.user_profile.hometown,？） establish , The query again ？ Situated ?e2; If not , Just variables e2 empty . Some of the results are shown below ：

so Results and 2.2 The conditional query of a single triplet is known , The results are almost consistent —— But in the end, there were more e2 A column of .

2.6 Any one of them can be satisfied UNION

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?e1 ?r ?e2
WHERE {
{ ?e1 ?r ns:m.0160w } UNION { ?e1 ns:freebase.user_profile.hometown ?e2 }
}

so , Meet the three elements Group condition 1(?e1 ?r ns:m.0160w) Or conditions 2(?e1 ns:freebase.user_profile.hometown ?e2) Any one of , Just go back to . Some of the results are as follows ：

Conditions 1 UNION Conditions 2, and , Conditions 1 OPTIONAL Conditions 2 The difference is ：

1. UNION Characterization “ or ” Relationship , Requirements 1,2 among At least one of them Conditions
2. Conditions 1 OPTIONAL Conditions 2, Only required Conditions must be met 1; As for the conditions 2, If meet , The query is also executed incidentally .

2.7 Statistics count COUNT

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT COUNT DISTINCT ?e1 ?r ?e2
WHERE {
{ ?e1 ?r ns:m.0160w } UNION { ?e1 ns:freebase.user_profile.hometown ?e2 }
}

Here and DISTINCT In combination with , That is to say, the statistics of the non repeated e1,r,e2 Number of combinations , give the result as follows ：

2.8  Language qualification

PREFIX ns: <http://rdf.freebase.com/ns/>
SELECT DISTINCT ?e1 ?r
WHERE {
?e1 ?r ns:m.0160w .
?e1 ns:freebase.user_profile.hometown ?e2 .
FILTER(lang(?e1)!='zh')
}

combination FILTER Realization , Filter out ( As long as this part ） Language is not chinese （zh） Of Variable e1 and r.

The result is similar to 2.3 Nested queries are consistent . 

3、 Reference resources

Reference blog ：

1. sparql Summary of common query statements _TechOnly1988 The blog of -CSDN Blog _sparql intersection
2. sparql Brief introduction - Introduction to grammar _minosisterry The blog of -CSDN Blog _sparql grammar
3. sparql Brief introduction - Introduction to grammar _minosisterry The blog of -CSDN Blog _sparql grammar