A relational database collects different types of data sets that use tables, records, and columns. It is used to create a well-defined relationship between database tables so that relational databases can be easily stored. For example of relational databases such as Microsoft SQL Server, Oracle Database, MYSQL, etc. Show There are some important parameters of the relational database:
Employee table (Relation / Table Name) EmpIDEmpNameEmpAgeCountryNameEmp 101Andrew Mathew24USAEmp 102Marcus dugles27EnglandEmp 103Engidi Nathem28FranceEmp 104Jason Quilt21JapanEmp 108Robert29ItalyFollowing are the different types of relational database tables.
One to One Relationship (1:1): It is used to create a relationship between two tables in which a single row of the first table can only be related to one and only one records of a second table. Similarly, the row of a second table can also be related to anyone row of the first table. Following is the example to show a relational database, as shown below.  One to Many Relationship: It is used to create a relationship between two tables. Any single rows of the first table can be related to one or more rows of the second tables, but the rows of second tables can only relate to the only row in the first table. It is also known as a many to one relationship. Representation of One to Many relational databases: Representation of many to one relational database Many to Many Relationship: It is many to many relationships that create a relationship between two tables. Each record of the first table can relate to any records (or no records) in the second table. Similarly, each record of the second table can also relate to more than one record of the first table. It is also represented an N:N relationship. For example, there are many people involved in each project, and every person can involve more than one project. As we learned in the article on associations, objects interact with each other and we use relationships in our model of these objects to capture these interactions. In that section we learned about 1-to-many relationships. There are also many-to-many and one-to-one relationships; as you can see these three types of relationships are named after the cardinality constraints. There is a fourth type, the many-to-one relationship, however, that is just the reverse direction of the one-to-many relationship. This article introduces a design pattern for the many-to-many relationship, and shows the UML class diagram which is then mapped to the relational model. The sales databaseSo far our sales database has customers and orders. To finish the design pattern, we need products to sell. We’ll first describe what the Product class means, and how it is associated with the Order class. Below is how Product might be described A product is a specific type of item that we have for sale. Each product has a descriptive name; we distinguish similar products by the manufacturer’s name and model number. For each product, we need to know its unit list price and how many units of this product we have in stock. It is important to understand exactly what is meant by a product so that it can be modeled correctly. An example product (an instance of the Product class, a Product object) might be named “Blender, Commercial, 1.25 Qt.”, manufactured by Hamilton Beach, model number 908. This is a type of product, not an individual boxed blender that is sitting on our shelves. The same manufacturer probably has different blender models (909, 918, 919), and there are probably blenders that we stock that are made by other companies. Each such blender would be a different instance of this class. The following would be a typical description of how products relate with orders. An order can include many products, though a product will only be in an order once. Each time an order is placed for a product, we need to know how many units of that product are being ordered and the price we are actually selling the product for. The latter is needed as the sale price might vary from the list price by customer discount, special sale, price changes, etc. From the above description, we can extract the following sentences which focus on the constraints that will need to be modeled.
There are significant differences in this modeling problem compared to what we have seen before; these are listed below.
Class diagramThe many-to-many relationship will be modeled with an association that specifies a many cardinality in the multiplicity constraints of both ends of the association. UML gives us a way to model relationship attributes by showing them in an association class that is connected to the association by a dotted line. This is shown in the figure below with the OrderLine association class and the two attributes quantity and unitSalePrice as described above. If no information needs to be modeled about specific associations between objects (such as the quantity and unitSalePrice), then the UML class diagram omits the association class. Association classes can be attached to associations of different cardinalities, not just many-to-many associations as is the case with OrderLine. There are two additional attributes shown in the class diagram that we haven’t talked about yet. We need to know the subtotal for each order line (that is, the quantity times the unit sale price) and the total dollar value of each order (the sum of the subtotals for each line in that order). As just described, the values of these two attributes can be computed by using values in other attributes; in the class diagram we precede their name by a “/” to denote that they are derived attributes. Since these values can be computed or derived from the values of other attributes already in the database, they don’t need to be stored in the database, and they are not included in the relation scheme. In fact, they could be stored. The reader should consider the advantages and disadvantages of whether or not to store derived attributes in the database. Relation scheme diagramWe can’t represent a many-to-many association directly in a relation scheme, because two tables can’t be children of each other — there are no parent/child roles that can be assigned as both cardinalities are many. So for every many-to-many association, we will need an additional table, known as a junction table, in the database that will store all the relationship instances. The relation scheme of such junction tables are also part of the database scheme diagram. The scheme of a junction table (sometimes also called a join table or linking table) will contain two sets of FK attributes, one for each side of the association. If there is no association class, then the scheme only contains these two FKs. If there is an association class (as is the case here with OrderLines), its attributes will go into the junction table scheme. In general, to map a many-to-many association to the relational model, a junction table is introduced to the relational database scheme. The junction table includes two FKs, one to each of the related schema. The PK of this junction table is both FKs combined. If the many-to-many association has an association class, then its attributes are added to the junction table. These attributes of the association class have no influence on the PK of the junction table. The PK of a junction table that implements a many-to-many association is always and exactly the combination of the two FKs. In the relational model, the many-to-many association between Orders and Products has turned into a one-to-many relationship between Orders and Order Lines, plus a many-to-one relationship between Order Lines and Products. However, this does not mean the UML class diagram can be updated to show a one-to-many association and a many-to-one association with the respective classes. The UML class diagram given above is the correct model for the problem. The list below provides the English description matching these two relational model relationships along with what the constraints say about the data in those tables
Unique identifiersThe UPC is an external key. UPCs are defined for virtually all grocery and manufactured products by a commercial organization called the Uniform Code Council, Inc.® We will use it as the primary key of our Products table, which also has a candidate key here: With Orders now a parent of OrderLines, we might have decided that it needs a surrogate key (order number) to be copied into the OrderLines. In fact, most sales systems do this, as you know if you’ve ever tried to check on the status of something you’ve ordered from a company. For this example, it seems to be just as easy to stick with the existing PK of Orders, since it already has a surrogate key from Customers, and the order date doesn’t add much size. To uniquely identify each order line, we need to know both which order this line is contained in, and which product is being ordered on this line. The two FKs, from Orders and Products, together form the only candidate key of this relation and therefore the primary key. Data representationThe key to understanding how a many-to-many association is represented in the database is to realize that each row of the junction table (in this case, OrderLines) connects exactly one row from the left table (Orders) with exactly one row from the right table (Products). Each PK of Orders can be copied many times to OrderLines; each PK of Products can also be copied many times to OrderLines. But the same pair of FKs in OrderLines can only occur once. In the figure below we show only the PK and FK columns for sake of space. How is a manyMany-to-many relationships are implemented through an additional table that contains matching fields for both of the related tables. The formal term for a table is tuple. In a relation, the order of columns is immaterial. Rows in a table often are called records.
How is a manyMany-to-many (m:n) relationships add complexity and confusion to your model and to the application development process. The key to resolve m:n relationships is to separate the two entities and create two one-to-many (1:n) relationships between them with a third intersect entity.
Which goes in the table on the many side of a one many relationship?In a one-to-many relationship, the table on the one side of the relationship is the primary table and the table on the many side is the related table. A one-to-many relationship is the most common relationship found between tables in a relational database.
How can you define a manyA many-to-many relationship exists when one or more items in one table can have a relationship to one or more items in another table. For example: Your Order table contains orders placed by multiple customers (who are listed in the Customers table), and a customer may place more than one order.
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