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11.
Steffen Skatulla Ralf Schaarschmidt Peter Pistor Klaus Küspert 《Informatik - Forschung und Entwicklung》2000,22(2):161-170
Bei relationalen Datenbanksystemen haben sich de-jure-Normen für SQL stetig weiterentwickelt; mit diesen Normen wird es einfacher, portable Datenbankanwendungen zu entwickeln. Die SQL-Norm ruht auf zwei S?ulen, zum einen auf der Datenbanksprache SQL, zum anderen auf einer Vorgabe eines Datenbankkatalogs, welcher die gespeicherten Daten beschreiben soll. Im vorliegenden Beitrag wird derjenige Teilbereich des Datenbankkatalogs der aktuellen SQL99-Norm dargestellt, der bereits in der SQL92-Norm (ISO/IEC 9075:1992 bzw. DIN 66315) definiert ist und die klassischen (nicht objektrelationalen) Elemente einer relationalen Datenbank beschreibt. In dieser Darstellung nimmt der zweischichtige Spezifikationsansatz (Informationsschema, Definitionsschema) einigen Raum ein; mit seiner Hilfe weist SQL einen eleganten Weg, auf dem existierende Datenbanksysteme normkonforme Datenbankkataloge bereitstellen k?nnen. Anhand des Datenbankmanagementsystems DB2 wird anschlie?end untersucht, wie ein normkonformer Datenbankkatalog (das sogenannte Informationsschema) „oberhalb” des verfügbaren DB2-Katalogs realisiert werden kann, d. h. wie die vom Informationsschema geforderten Sichten auf DB2-Katalogtabellen abgebildet werden k?nnen. Auf generelle Schwierigkeiten und M?glichkeiten einer solchen Abbildung (ein- oder zweistufig) wird ebenso eingegangen wie auf Probleme, die sich speziell bei DB2 ergeben. In einer prototypischen Realisierung konnte das Informationsschema bis auf wenige Einschr?nkungen komplett umgesetzt werden. Damit wurde die Machbarkeit normkonformer Kataloge nachgewiesen. In heute verfügbaren DBMS-Produkten sind Datenbankkataloge in dieser konsequent normkonformen Gestalt leider immer noch nicht anzutreffen. 相似文献
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Tim De Feyter Ralf Caers Claudia Vigna Dries Berings 《Learning and individual differences》2012,22(4):439-448
The main purpose of this study is to unravel the impact of the Big Five personality factors on academic performance. We propose a theoretical model with conditional indirect effects of the Big Five personality factors on academic performance through their impact upon academic motivation. To clarify the mixed results of previous studies concerning the impact of neuroticism, we suggest a moderating role of self-efficacy. Hierarchical, moderated mediation and mediated moderation regression analyses were performed on longitudinal data collected from 375 students of a University college in Belgium. The findings revealed a positive indirect effect of neuroticism on academic performance at higher levels of self-efficacy, complemented by a positive direct effect of neuroticism at lower levels of self-efficacy. Finally, this study showed that conscientiousness positively affected academic performance indirectly through academic motivation, but also that it is a condition for the indirect impact of extraversion, neuroticism, and conscientiousness. 相似文献
13.
Maria Magdalena Isac Ralf Maslowski Greetje van der Werf 《School Effectiveness & School Improvement》2013,24(3):313-333
In this study, a comprehensive educational effectiveness model is tested in relation to student's civic knowledge. Multilevel analysis was applied on the dataset of the IEA Civic Education Study (CIVED; Torney-Purta, Lehmann, Oswald, & Schulz, 2001), which was conducted among junior secondary-school students (age 14), their schools, and their teachers. In total, 28 countries, 4,136 classrooms, and 93,565 students were included in the analysis. The results indicated that the influences on students' civic knowledge are multilevel. Students' civic knowledge and skills were partially explained by individual characteristics, by factors related to quality and opportunities for civic learning offered by classrooms and class composition, and by factors at the national context level. We conclude that most effectiveness factors are relevant for the field of civic and citizenship education and that schooling and educational policy matter for students' success in this field. 相似文献
14.
Chronic morphine drinking establishes morphine tolerance, but not addiction in Wistar rats 总被引:1,自引:0,他引:1
Binsack R Zheng ML Zhang ZS Yang L Zhu YP 《Journal of Zhejiang University. Science. B》2006,7(11):892-898
Objective: Some animal models apply morphine in the drinking water to generate addiction, but related reports are not free of conflicting results. Accordingly, this study aimed to figure out if chronic consumption of morphine in the drinking water can induce morphine addiction in Wistar rats. Methods: For 3 weeks, the animals received a daily morphine dose of 35 mg/kg by offering a calculated volume of sugar water (5% sucrose) with morphine (0. l mg/ml) to each rat; animals receiving just sugar water served as controls. Immediately after the treatment phase, the tail immersion test was used to check for morphine tolerance, and all animals were then kept on tap water for one week (withdrawal phase). Afterwards, all rats were allowed to choose their drinking source by offering two bottles, containing sugar water without and with morphine, simultaneously for two days (preference phase). Results: While the chronic consumption of morphine led to a reduction in body weight and to morphine tolerance, the morphine-treated Wistar rats did not show any preference for the opiate-containing sugar water. Conclusion: Body weight loss and tolerance do not reveal a condition of drug craving, and current animal models should be re-evaluated regarding their potential to establish morphine addicted animals. 相似文献
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The authors critique the mechanistic notion of aspirations running through much research and policy‐making on educational and vocational outcomes. They present a performative model, with individuals drawing on limited social resources to express aspirations within constrained contexts. This argument is illustrated by discussion of the findings of large‐scale empirical investigation of the aspirations of 490 young people in three UK schools. Five themes from this analysis are presented and it is argued that these need to be explored in order to enrich and expand our understanding of young people's expression of aspirations. 相似文献
18.
Gianluca Demartini Claudiu S. Firan Tereza Iofciu Ralf Krestel Wolfgang Nejdl 《Information Retrieval》2010,13(5):534-567
Entity Retrieval (ER)—in comparison to classical search—aims at finding individual entities instead of relevant documents.
Finding a list of entities requires therefore techniques different to classical search engines. In this paper, we present
a model to describe entities more formally and how an ER system can be build on top of it. We compare different approaches
designed for finding entities in Wikipedia and report on results using standard test collections. An analysis of entity-centric
queries reveals different aspects and problems related to ER and shows limitations of current systems performing ER with Wikipedia.
It also indicates which approaches are suitable for which kinds of queries. 相似文献
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