Partial Dependency in DBMS

Partial dependency is a concеpt in database dеsign that describes how a non-kеy attributе depends on only a part of thе primary kеy. Database management systеms (DBMS) arе softwarе applications that storе, manipulatе, and rеtriеvе data in a structured and organizеd way. It managеs data in tablеs.

• A tablе has attributеs, which arе thе data propеrtiеs.
• A kеy is an attributе or a group of attributеs that can idеntify еach row in a tablе.
• A primary kеy is thе main kеy of thе tablе, and a candidatе kеy is a possiblе primary kеy.
• A non-kеy attributе is an attributе that is not part of any kеy.
• For еxamplе, a tablе of students may havе StudеntID as thе primary kеy, Namе and Agе as candidatе kеys, and Coursе as a non-kеy attributе. 

Keyterms: Partial dependency, Database, Primary key, Candidatе kеy, Database management systеms, Non-key attribute, Possible Primary Key

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What is Partial Dependency?

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Partial dependency is a concept in database design that describes how a non-key attribute depends on only a part of the primary key. A non-key attribute is an attribute that is not part of the primary key, which is a set of attributes that uniquely identifies each record in a table.

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How is Partial Dependency Identified?

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To identify partial dependency, onе can usе two ways:

• Onе way to identify Partial Dependency is to analyse thе functional dependencies between thе attributes of a tablе.
• Functional dependencies dеscribе how onе attributе or a group of attributеs dеtеrminеs anothеr attributе.
• If an attributе is dеtеrminеd by only part of thе primary kеy, thеn it has a partial dependency. 

For example, in a table of students, if the primary key is StudentID and CourseID, and the non-key attributes are Name, Age, and Grade, then Name and Age have partial dependencies on the primary key, because they are determined by only StudentID, not by both StudentID and CourseID.

• Another way to identify partial dependency is to look for redundant data in a table. 
• If the same data is repeated for multiple rows in a table, then it is likely that there is partial dependency. 
• Redundant data can lead to data inconsistency and anomalies, which can be problematic in database management. 

For example, in a table of orders, if the primary key is OrderID and ProductID, and the non-key attributes are CustomerID, OrderDate, and Quantity, then CustomerID and OrderDate have partial dependencies on the primary key, because they are repeated for every product in the same order.

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What Causes Partial Dependency to Occur?

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Partial dependency is caused by:

• Having a table with a primary key that has more than one attribute
• Having a non-key attribute that relies on only part of the primary key, not the whole key
• Having a table that represents a many-to-many relationship between two entities in a database

For example, in the “Orders” table, the primary key is OrderID and ProductID, which are both candidate keys. However, some non-key attributes, such as CustomerID and OrderDate, depend on only OrderID, which is a part of the primary key.

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When does Partial Dependency occur?

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Partial dependency occurs when a non-key attribute of a table depends on only a part of the primary key, rather than the whole key. This means that the value of the non-key attribute can be determined by a subset of the primary key attributes, but not by the entire key.

1. Partial dependency can lead to data redundancy and inconsistency, which can affect the performance and integrity of the database.
2. To avoid partial dependency, the table should be normalized to the second normal form (2NF), which requires that every non-key attribute is fully dependent on the primary key.

Lets understand this using an example:

Here is a table called “Employees” that stores information about employees and their departments:

• The primary key of this table is EmployeeID, which is a unique identifier for each employee. The non-key attributes are Name, DepartmentID, DepartmentName, and Salary.
• Partial dependency occurs when a non-key attribute depends on only a part of the primary key, not the whole key. In this table, we can see that DepartmentName depends on DepartmentID, which is a part of the primary key. 
• This means that DepartmentName has a partial dependency on the primary key. This can be written as:

Department ID → Department Name

• This partial dependency causes redundancy and inconsistency in the table. 
• For example, the DepartmentName “Sales” is repeated twice for different employees. If we want to change the name of the department, we have to update multiple rows in the table. This can lead to errors and anomalies.
• To minimize partial dependency, we can apply the second normal form (2NF) of normalization, which requires that every non-key attribute must be fully dependent on the primary key. 
• To achieve this, split the table into two tables, one for employees and one for departments, as follows:

Now, each non-key attribute is fully dependent on the primary key of its table, and there is no partial dependency. The redundancy and inconsistency are also eliminated, and the data is more consistent and efficient.

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Benefits of Addressing Partial Dependency in Database Design

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Benefits of addressing partial dependency are mentioned below:

• One can eliminate redundancy and anomalies in the table.
• Second normal form (2NF) removes partial dependency by splitting the table into two or more tables, each with a single theme or entity.
• Splitting the table improves the performance and efficiency of the database by reducing the size and complexity of the tables and enabling easy data retrieval and validation.
• Splitting the table also enhances the security and maintainability of the database by allowing different access rights and privileges for different tables and making the data more flexible and adaptable to changes.

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Things to Remember

• Partial dependency is a situation where a non-key attribute depends on a subset of the primary key, not the entire key.
• It is a bad database design that causes data problems and inefficiency. 
• Partial dependency can be found by checking the functional dependencies or the redundant data in a table. 
• Partial dependency happens when a table has a primary key with more than one attribute. 
• Partial dependency means a table is in 1NF but not in 2NF. Partial dependency can be fixed by breaking the table into smaller tables. 
• Partial dependency is not the same as full functional dependency, which is when a non-key attribute relies on the whole primary key.
• Full functional dependency is good and should be kept.

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