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Most NoSQL systems are schema-on-read: data can be stored without first having to declare a Schema that imposes a structure. This schemaless feature offers flexibility to evolve data-intensive applications when data frequently change. However, freeing from declaring schemas does not mean their absence, but rather that they are implicit in data and code. Therefore, diagramming tools similar to those available for relational systems are also needed to help developers and administrators understanding NoSQL schemas. Visualizing diagrams is not practical if schemas contain hundreds of database entities, and exploration or query facilities are then needed. In schemaless NoSQL stores, data of the same entity can be stored with different structure which can increase the difficulty of having readable diagrams. NoSQL schema management tools should therefore have three main components: schema extraction, schema visualization, and schema query. Since that there exist four main NoSQL data models, it is convenient that such tools can be built on a generic data model that provide platform-independence to query and visualize schemas. With the aim of favoring the creation of generic database tools, the authors of this paper defined the U-Schema unified data model that integrates the four main NoSQL data models and the relational model. This paper is focused on querying NoSQL and relational schemas which are represented as U-Schema models. We present the SkiQL language designed on U-Schema to achieve a platform-independent schema query service. SkiQL provides two constructs: schema-query and relationship-query. The former allows to obtain information of entity or relationship types, and the latter that of the aggregations or references (relations among types). We will show how SkiQL was evaluated by calculating well-known metrics for languages and using a survey.