Introduction to Database True or False - Database Questions and Answers
Mar 22, A relationship is established between two database tables when one helps ensure data integrity, contributing to database normalization. This is the database questions and answers section on "Introduction to Database True or False" with explanation for 4. A database has data and relationships. Most organizations build several databases, each one sharing data with .. by the DBMS to physically establish record types, fields, and structural relationships.
Example of one-to-many relationship. One-to-Many relationships can also be viewed as Many-to-One relationships, depending on which way you look at it. Each customer can only be assigned one city. One city can be assigned to many customers. Many-to-Many In a many-to-many relationship, a row in table A can have many matching rows in table B, and vice versa.
A many-to-many relationship could be thought of as two one-to-many relationships, linked by an intermediary table. This table is used to link the other two tables together.
It does this by having two fields that reference the primary key of each of the other two tables. The following is an example of a many-to-many relationship: This is the Relationships tab that is displayed when you create a relationship Microsoft Access. In this case, a many-to-many relationship has just been created.
The Orders table is a junction table that cross-references the Customers table with the Products table. So in order to create a many-to-many relationship between the Customers table and the Products table, we created a new table called Orders. The values that these fields contain should correspond with a value in the corresponding field in the referenced table. So any given value in Orders.
CustomerId should also exist in the Customer. Relations can be modified using the insert, delete, and update operators.
New tuples can supply explicit values or be derived from a query. Similarly, queries identify tuples for updating or deleting. Tuples by definition are unique. If the tuple contains a candidate or primary key then obviously it is unique; however, a primary key need not be defined for a row or record to be a tuple.
The definition of a tuple requires that it be unique, but does not require a primary key to be defined. Because a tuple is unique, its attributes by definition constitute a superkey. Base and derived relations[ edit ] Main articles: Relvar and View database In a relational database, all data are stored and accessed via relations. Relations that store data are called "base relations", and in implementations are called "tables".
Other relations do not store data, but are computed by applying relational operations to other relations. These relations are sometimes called "derived relations".
Database - Introduction to Database
In implementations these are called " views " or "queries". Derived relations are convenient in that they act as a single relation, even though they may grab information from several relations.
Also, derived relations can be used as an abstraction layer. Mathematically, attaching a domain to an attribute means that any value for the attribute must be an element of the specified set. The character string "ABC", for instance, is not in the integer domain, but the integer value is. Another example of domain describes the possible values for the field "CoinFace" as "Heads","Tails". So, the field "CoinFace" will not accept input values like 0,1 or H,T. Constraints[ edit ] Constraints make it possible to further restrict the domain of an attribute.
For instance, a constraint can restrict a given integer attribute to values between 1 and Constraints provide one method of implementing business rules in the database and support subsequent data use within the application layer.
SQL implements constraint functionality in the form of check constraints. Constraints restrict the data that can be stored in relations. These are usually defined using expressions that result in a boolean value, indicating whether or not the data satisfies the constraint.
Constraints can apply to single attributes, to a tuple restricting combinations of attributes or to an entire relation. Since every attribute has an associated domain, there are constraints domain constraints. The two principal rules for the relational model are known as entity integrity and referential integrity. Unique key A primary key uniquely specifies a tuple within a table. In order for an attribute to be a good primary key it must not repeat.
While natural attributes attributes used to describe the data being entered are sometimes good primary keys, surrogate keys are often used instead. A surrogate key is an artificial attribute assigned to an object which uniquely identifies it for instance, in a table of information about students at a school they might all be assigned a student ID in order to differentiate them. The surrogate key has no intrinsic inherent meaning, but rather is useful through its ability to uniquely identify a tuple.
Another common occurrence, especially in regard to N: M cardinality is the composite key. A composite key is a key made up of two or more attributes within a table that together uniquely identify a record. For example, in a database relating students, teachers, and classes. Classes could be uniquely identified by a composite key of their room number and time slot, since no other class could have exactly the same combination of attributes.
The 3 Types of Relationships in Database Design | animesost.info
In fact, use of a composite key such as this can be a form of data verificationalbeit a weak one. Foreign key A foreign key is a field in a relational table that matches the primary key column of another table. The foreign key can be used to cross-reference tables.