Update on 21 Mar, 23
A vast quantity of information is out there in this real world. Now, for storing the information in the database management system, an oversized variety of tables or needed.
These tables could contain thousands of duplicates, sorted, and unsorted Records. Now, fetching any specific or specific record with no constraints or restrictions from these tables may be a tough method.
To beat all the difficulties, a brand new thought of Keys arose. Keys make sure that there aren't any rows or tuples with duplicate records. Let’s perceive the thought of Keys in the database management system.
1. Primary key
2. Candidate key
3. Super Key
4. Foreign key
5. Alternate key
6. Composite key
7. Artificial key
It is the primary key won't to determining one associate degree only 1 instance of an entity unambiguously. associate degree entity will contain multiple keys, as we tend to see within the PERSON table.
The key that is best suited from those lists becomes a primary key. within the worker table, ID may be the first key since it's distinctive for every worker.
Within the worker table, we will choose the identification number and Passport_Number as primary keys since they're also distinctive. For each entity, the first key choice relies on needs and developers.
A candidate secret's associate degree attribute or set of attributes that may unambiguously determine a tuple.
Except for the first key, the remaining attributes are thought-about candidate keys. The candidate keys are as robust because the primary key.
For example: within the worker table, the id is best fitted to the first key. the remainder of the attributes, like SSN, Passport_Number, identification number, etc., are thought-about a candidate key.
Super secret's associate degree attribute set may unambiguously determine a tuple. a brilliant secret's a superset of a candidate key.
For example: within the worker table, for(EMPLOEE_ID, EMPLOYEE_NAME), the name of 2 workers may be identical, however, their EMPLYEE_ID cannot be identical. Hence, this mix may also be a key.
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The super key would be EMPLOYEE-ID (EMPLOYEE_ID, EMPLOYEE-NAME), etc.
Foreign keys are the column of the table that won't purpose to the first key of another table. each worker works in an exceedingly specific department in an exceeding company, and worker and department are 2 d entities.
So, we won't store the department's data within the worker table. that is why we tend to link these 2 tables through the first key of 1 table.
We tend to add the first key of the DEPARTMENT table, Department_Id, as a brand new attribute within the worker table.
In the worker table, Department_Id is the foreign key, and each of the tables is connected.
There could also be one or additional attributes or a mix of attributes that unambiguously determine every tuple in an exceeding relation.
These attributes or combos of the attributes are referred to as the candidate keys. One secret's chosen because the primary key from these candidate keys, and also the remaining candidate key, if it exists, is termed the alternate key. In alternative words, the full variety of the alternate keys is the total variety of candidate keys minus the first key.
The alternate key could or might not exist. If there's only 1 candidate key in an exceeding relation, it doesn't have an associate degree alternate key.
Example: worker relation has 2 attributes, Employee_Id and PAN_No, that act as candidate keys.
During this relation, Employee_Id is chosen because the primary key, the alternative candidate key, PAN_No, acts because of the Alternate key.
Whenever a primary key consists of quite one attribute, it's referred to as a composite key. This secret's additionally referred to as Concatenated Key.
As an example, in worker relations, we tend to assume that an associate degree worker could also be assigned multiple roles, associate degree workers may match on multiple comes at the same time.
So, the first key is composed of all 3 attributes, specifically Emp_ID, Emp_role, and Proj_ID together. So, these attributes act as a composite key since the first key contains quite one attribute.
The key-created exploitation of haphazardly assigned knowledge is referred to as artificial keys. These keys are created once a primary secret giant and complicated and have no relationship with several alternative relations. the information values of the synthetic keys are typically numbered in an exceedingly serial order.
For example, the first key, which consists of Emp_ID, Emp_role, and Proj_ID, is a giant in worker relations. So, it'd be higher to feature a brand new virtual attribute to spot every tuple within the relation unambiguously.
Generalization is sort of a bottom-up approach during which 2 or additional entities of lower-level mix to make a higher-level entity if they need some attributes in common. In generalization, the associate degree entity of the next level may also mix with the entities of the lower level to make an extra higher-level entity.
Generalization is additional to taxonomic group and taxon system, however, the sole distinction is the approach. Generalization uses the bottom-up approach.
In generalization, entities are combined to make an additional generalized entity, i.e., subclasses are combined to create a taxon.
For example, college and Student entities may be generalized and make a higher-level entity, Person.
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