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A database model provides the means for specifying particular data structures, for constraining the data sets associated with these structures, and for manipulating the data. The specification of structure and constraints is done using a data definition language (DDL), and the specification of manipulation is done using a data manipulation language (DML). The most prominent structures that have been used for databases to date are graphs in the network, semantic, and object-oriented models; trees in the hierarchical model; and relations in the relational model.
One-to-many the most common type of relationship is one-to-many. This means that for each occurrence in each entity, the parent entity, there may be one or more occurrences in a second entity, the child entity, to which it is related. (Silberschatz, Korth and Sudarshan, 2011)
The One-To-Many Model One-to-many the most common type of relationship is one-to-many. This means that for each occurrence in each entity, the parent entity, there may be one or more occurrences in a second entity, the child entity, to which it is related.
The One-to-One Model In a one-to-one relationship, a row in a table is related to only one or none of the rows in a second table. These relationships are not as common as one-to-many relationships, because if one entity has an occurrence for a corresponding row in another entity, in most cases, the attributes from both entities should be in a single entity. Note that in a one-to-one model, the concepts of child and parent tables are interchangeable. Each table relies on the other to form the complete picture of each patient or employee. (Hernandez, 2013)
The Many-to-Many Model In a many-to-many relationship, one row of a table may be related to many rows of another table, and vice versa. Usually, when this relationship is implemented in the database, a third entity is defined as an intersection table to contain the associations between the two entities in the relationship.
In database tables, each column or attribute describes some piece of data that each record in the table has. The terms column and attribute are used fairly interchangeably, but a column is really part of a table, whereas an attribute relates to the real-world entity that the table is modeling. In Figure 2 you can see that each employee has an employeeID, a name, a job, and a departmentID. These are the columns of the employee table, sometimes also called the attributes of the employee table.
Look again at the employee table. Each row in the table represents a single employee record. You may hear these called rows, records, or tuples. Each row in the table consists of a value for each column in the table.
Keys are a very important concept in a successful database design. Keys come in five basic flavours: Super Keys, Candidate Keys, Primary Keys, Foreign Keys, and Keys. Note: This is not a full explanation of keys and how to use them, there is a section on keys later in this document. A Super Key is a column (or set of columns) that can be used to identify a row in a table. A Key is a minimal Super Key. For example, look at the employee table. We could use the employeeID and the name together to identify any row in the table. We could also use the set of all the columns (employeeID, name, job, departmentID). These are both Super Keys.
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