Which of the following is the term for a key that contains more than one attribute more than one column )?
Definition of Composite key: A key that has more than one attributes is known as composite key. It is also known as compound key. Note: Any key such as super key, primary key, candidate key etc.
What is a well formed relation?
In mathematics, a binary relation R is called well–founded (or wellfounded) on a class X if every non-empty subset S ⊆ X has a minimal element with respect to R, that is, an element m not related by sRm (for instance, “s is not smaller than m”) for any s ∈ S. In other words, a relation is well founded if.
What relationship database principle ensures that a table with a foreign key does not have rows with foreign key values that do not have matching primary key values in the related table?
Referential integrity requires that a foreign key must have a matching primary key or it must be null. This constraint is specified between two tables (parent and child); it maintains the correspondence between rows in these tables.
Can foreign key be null?
Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). Null by definition is not a value.
What is primary key short answer?
In the relational model of databases, a primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table). Informally, a primary key is “which attributes identify a record”, and in simple cases are simply a single attribute: a unique id.
What is a repeating group?
A repeating group is a series of fields/attributes that are repeated throughout a database table. It is a common problem faced by organizations both large and small, one that can have several ramifications.
Which of the following is a component of a database system?
The four components in a database system are: the user, the database application, the database management system (DBMS) and the database. The database stores the user data, the database metadata, indexes, triggers, stored procedures and application metadata.
Can be used to build prototype databases?
46) Microsoft Access can be used to build a prototype database, which allows users: Access forms and reports are very simplistic.
How foreign key is represent in ER diagram?
The ER model does not use foreign keys to represent relationships. It uses lines between boxes. The lines have some kind of indicator for cardinality at either end or both ends. Sometimes, a relationship will be indicated separately by a diamond.
Can a primary key be a foreign key?
Yes, it is legal to have a primary key being a foreign key. This is a rare construct, but it applies for: a 1:1 relation. The two tables cannot be merged in one because of different permissions and privileges only apply at table level (as of 2017, such a database would be odd).
Why foreign key is called referential integrity?
Since the purpose of the foreign key is to identify a particular row of referenced table, it is generally required that the foreign key is equal to the candidate key in some row of the primary table, or else have no value (the NULL value.). This rule is called a referential integrity constraint between the two tables.
Why is my foreign key null?
This is entirely independent of whether the column is referenced in a foreign key constraint. The value in the column is NULL because that’s the value that was assigned when the row was inserted. The value was assigned, whether it was explicitly set, or whether it was derived from the default value for the column.
How do you set a foreign key to null?
- you can edit database field and allow null value to be insert. –
- To say that a field is a foreign key means it is constrained to match a primary key in the specified table.
- @hardmath..So it cannot be Null in that case (as matching makes no sense with Null values)..
Can foreign key be not unique?
BUT, as it is the Primary Key of another table, it must be unique in this table. No. But the values must exists first on the parent table before you can insert it on the table. No, foreign keys do not have to be unique.