Entity relationship xml schema

Generating a Customized XML Schema - ER/Studio Data Architect

entity relationship xml schema

One of {contractemployee} is expected. What changes shall I make in my code to correctly implement the xml document from the given image. What is a good XML schema for a relational schema? Entity Relationship (ER) Model. ○ Structures: Entity Types, Relationship Types. We propose a mapping from the Enhanced Entity Relationship conceptual model to the W3C XML Schema Language with the following properties: information.

To create more import declarations, repeat steps 1 to 3. You can edit the name of the import declaration or delete it by right-clicking its name under the Import node and then selecting Edit or Delete. Dragging a domain that contains a subdomain into the Simple Types node does not include any of its subdomains in the schema.

From the list of Available Objects, click a simple domain or attribute and then drag it to the Simple Types node in the Schema Body pane. The Simple Types node expands to show the newly added object. If the target domain is for the simple type added, you can select the schema element, scroll through the properties, and change the default type and type prefix by clicking the Property Value list. The available namespaces are dependent on the namespaces declared on page 2 of the wizard.

The reference values for the object appear as a property of the object in the Schema Body.

To preview the schema, click Preview or for more information, see Preview the Schema. The Elements node expands to show the newly added objects. Notes You cannot drag reference values onto domains in the Elements node; instead, use the Simple Types node.

You can drag a simple entity or a complex entity, with all their attributes, to the Elements node and then drag other attributes to its choice, sequence, or all attribute container. To convert an attribute of an element to an element, drag the attribute out of the complexType container.

To convert an element to an attribute of another element, drag the element into the complexType node. An element defined in the elements node may have a complex type underneath it.

You can insert the complex type under the element and insert composites under that. If you drag an attribute to the complex type node under an element, it is included within the xs: If you drag an attribute to a composite node, it is included within the xs: The Groups node expands to show the newly added object.

Use the Groups node to define groups that another part of the schema, such as an element can reference. A group may have a composite underneath it just like an element or complex type. The Complex Types node expands to show the newly added object. Notes If the target domain is for the simple type added, you can select the schema element, scroll through the properties, and change the default type and type prefix by clicking the Property Value list.

Use the Complex Types node to define standalone complex types for later reuse. Typically, complex types define complex structures that elements in the element node can reference.

Complex types promote reuse in an XSD. The correctness of our approach is discussed in Section 4, where we give a soundness and correctness proof for the Xere mapping algorithm.

The information reported here is not complete, but enough for the reader to understand the Xere mapping and its implementation. Well—formed XML documents must obey to some simple rules, such as, e.

XML documents that are consistent w. As an example, Fig. The idea of XML comes from the success of markup languages like HTML, fo- cusing on a particular class of documents that show a strong hierarchical structure. The number of supporting technologies developed around XML made its success in few years. In the last years, XML languages are being adopted also for data description and manipulation. Indeed, the hierarchical structure of markup documents is very suitable to represent a wide variety of data, especially objects and similar struc- tures.

XML fragments are used to contain data structures and make them more portable and open—format. XSD distinguishes among simple and complex types: Content models are created by freely composing three basic models: A cardinality constraint can be assigned both to the elements and to the basic models, through the minOccurs and maxOccurs attributes. The element E contains a sequence of at least one element F, optionally followed by an element G or H.

Element F contains an integer, element G contains a string and element H contains a value belonging to the user—defined type a. Types can be derived by extension or restriction.

entity relationship xml schema

Elements of derived types can be de- clared compatible and arranged in an object-oriented fashion by means of substitu- tion groups.

A complex type derived by extension For instance, Fig. Schemas allow declarations of types, elements and attributes to be global or local, i. In the former case, the declared type, element or attribute has a distinctive name and can be referenced possibly many times in other parts of the schema. Both key and unique constraints have a scope, i. Schema keys and key references can be very complex. As an example, in Fig. The element C references the key A Key through its attributes k1 and k2.

ERD is a graphical model, i. The basic ERD elements, shown in Fig. They are depicted as diamonds connected through lines to the associated entities. Relationship symbols also contain the association name. They are written as min, max integer pairs close to the line that connects each entity with each relation. Likewise, attributes can be assigned a cardinality too.

They are represented by ellipses that can be attached to any entity or relationship symbol. The attribute name is inside or close to the related ellipse. If one or more attributes constitute the primary key for an entity, i. This means that the entity instances can be unambiguously distinguished only if they are considered within the context given by the associated entities.

As depicted in Fig. Generalizations can be total i. Before proceeding with the technical discussion, we present the motivating ideas behind the approach. The mapping should be able to preserve all the struc- tural information provided by an arbitrary Schema without including additional information which is not expressly given. In other words, the translation must not disregard relevant pieces of information and at the same time the process does not have to introduce additional information as part of the diagram.

In contrast with ER diagrams, Schemas are quite complete docu- ments which covers the most oddly things, from attributes to recursive types. Thus Schemas are located on a much lower abstraction level than ER models, which are usually intended as an intermediate design document.

In fact, not all the constructs used in the Schemas have a precise counterpart in the ER model, which is much simpler. Since there is no corre- sponding construct in the ER model, the generalization is used to gain the same result. Thus, the simulation of the choice is obtained by means of auxiliary entities used to represent the root of the generalization hierarchy and the specializations.

Most of such situations can be directly handled only at the logical e.

Interoperability mapping from XML schemas to ER diagrams | Alfonso Pierantonio - angelfirenm.info

DBMS level, as for instance, for attribute types and default values, respectively. To handle this information, we use an extended ER model, where the properties that cannot be embedded in the graphical model are ACM Transactions on Internet Technology, Vol.

entity relationship xml schema

Mapping of a simple sequence model. Consequently, any well-formed XML fragment and attribute meets this requirement, respectively. This form of genericity does not have a correspondence in the theory and practice of databases; therefore, it has been not included in the current form of the mapping.

This is used to specify type information about external entities, typically binary objects. For instance, it could be used to declare which application should handle a particular data format. This kind of details does not need to be stored in a database, since it goes beyond the data model purposes.

As in the previous case, these features do not need to be covered. These directives describe how to compose sev- eral Schemas together, possibly sharing some common features.

XML Schemas - XSD with Example - Difference between DTD and XSD in Hindi

However, since these directives do not provide any additional expressiveness, we left them out in this version of the Xere algorithm. In the following, we list the supported Schema features and describe how they are mapped to the ER model.

Each Schema element becomes an ER entity. The entity names are based on the corresponding element names. Elements in XML Schemas have a scope of visibility given by their nesting level, so that e. In this case, the entity name is obtained by appending the element name to the parent local name: The element primary key is chosen using the following rules: If an element is declared with the nillable attribute set to true, then an appropriate annotation is added to the ER diagram to remark that in the corresponding entity all the at- tributes can be set to null this is an important information for the DBMS.

The element processing continues mapping the element type. The corresponding mapping rules are described in the following.

It is important to retain such an information for later use, since simple types cor- respond to SQL types and restricted simple types can be expressed using SQL domains. Complex types are not mapped directly.

The complex type is recursively analyzed and the mapping algorithm is applied to its content usually a content model. Types derived by restriction are handled as natural by rewriting the entire complex type with appropriate restrictions. All content models i. Content model children can be either elements, other content models or the group element used to modularize frequently used content models. The mapping is designed to be as natural as possible and preserve most of the information given by the Schema.

In the following, we write content owner to denote the entity corresponding to the Schema element that declares a particular content model, and content children to denote the elements nested directly inside the content model. The content children are mapped to an ER fragment by applying recursively the Xere mapping algorithm.

Mapping of a choice model relationship in the XML Schema and thus they are oriented: For this reason, each ER fragment generated by the Xere algorithm can be seen as a tree with a distinguished root entity. The particular mapping of each content model is described in the following para- graphs. All the sequence children are recursively mapped to ER frag- ments. Then, the root entity of each fragment is connected to the content owner using a relation.

For child elements having a multiplicity maxOccurs greater than onethe instance ordering is expressed in the ER diagram by adding a position attribute to the corresponding relations see Fig. The choice model is translated using the generalization construct of ER diagrams. An auxiliary entity is attached to the content owner with an appropriate relation, and is specialized in as many other auxiliary entities as the content children are.

The reason for adding these auxiliary entities is that we need the corresponding relations to store the cardinality constraints. Finally, the ER fragments obtained from each content model children are connected to the corresponding auxiliary entity using a relation. Mapping of a substitution group.

Note that the ER generalization construct may not be always semantically con- sistent with the choice XML Schema content model. The all model is realized similarly to the sequence model, without keeping track of the children position in the sequence.

Mixed content models contain both elements and plain text. To handle this content, we modify the content model so that a special distinct text element is allowed anywhere plain text can appear.

In this way, we can apply the standard mapping algorithm and preserve the ordering between text and elements. The entity corresponding to the text element is created as for a global element, so that all the mixed content models refer to the same entity for storing text fragments. Global declarations and references. Elements, attributes and types can be glob- ally declared and then referenced by name. The algorithm handles both situations.

Substitution groups are translated using the generalization construct of ER diagrams see Fig. The actual entities corresponding to each element in the substitution group of A, including A itself, are attached to A SubstGroup as a specialization.

entity relationship xml schema

Finally, every time an element belonging to the substitution group of A is referenced in the schema, that reference is substituted with a reference to A SubstGroup see Fig.

Mapping of a sequence model with occurrence constraints Occurrence constraints. Occurrence constraints in content model children are mapped to relation cardinalities in the ER model. The cardinality is placed on the appropriate ER relation created during the content model mapping. A special attribute position is added to the relations to keep track of the occurrences ordinal position in choice and sequence content models when child elements have multi- plicity. If the occurrence constraint is placed on a sequence model, an auxiliary entity and a relation are created to store the corresponding cardinality see Fig.

Note that the role of auxiliary entities is only to provide a more natural way to map the Schema to the ER diagram. Most of the times, these temporary entities are discarded when reducing the ER model to a relational model. Therefore, storing such value in each entity instance would be a waste of space.

At the ER level we add an appropriate annotation to the diagram, so that the attribute could be handled in a special way when the ER is translated in a logical model and then deployed on a DBMS. If the key contains values or attributes from other child elements, we classify the entity as a weak entity.

This means that the entity primary key can be composed by a combination of 1 entity attributes and 2 external attributes that form the primary key of one or more entities that are attached to the current one through a relation. A simple example is shown in Fig. The new relation, which is included in the key, is named Key XX, where XX is the name of the external entity, to remind the designer that it has been created due to a key mapping.

However, note that the external entity attributes must also form a key for that entity: However, ER diagrams and the database theory in general say that each database table can have only one primary key.

Key references are mapped to the ER diagrams by creating new relations.

Indeed, keyref elements are used in XSD to declare relations between data that cannot be captured only using the document hierarchy. The new relations are named adequately, as shown in Fig.

Optimization of nested elements with simple type.

entity relationship xml schema

Finally, unique constraints are annotated to the diagram, since they cannot be expressed in the ER model, but are implemented at a lower DBMS level. The mapping described here has the advantage of making explicit the key and keyref semantics, which is rather complex in the XSD documents, through the easy constructs of the ER diagrams.

We see three major optimizations to the current Xere mapping algorithm: The algorithm in Section 3. However, elements with sim- ple type contain only a content attribute.

Actually such elements can be seen as particular attributes of the parent element: Schema designers often use local, nested elements with simple types instead of simple attributes only to give the XML documents a more modular and better-looking structure.

This suggests to translate such kind of element to an ER attribute. If the element has a multiplicity minOccurs or maxOccursthen we can use attributes with cardi- nality.