PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX textbook: <https://w3id.org/jp-textbook/>
PREFIX schema: <http://schema.org/>

SELECT * WHERE {
?s a textbook:TeachingUnit.
?s schema:name ?title.
?s textbook:school ?school .
?s schema:workExample/schema:isPartOf ?textbook .
?s schema:workExample/schema:pagination ?page .
 optional { ?s textbook:unit1 ?unit1 }.
 optional { ?s textbook:unit2 ?unit2 }.
 optional { ?s textbook:unit3 ?unit3 }.
 optional { ?s textbook:unit4 ?unit4 }.
}