Difference between Schema and Instance in DBMS

In a Database Management System (DBMS), the schema refers to the overall design or blueprint of the database, describing its structure (like tables, columns, and relationships). It remains relatively stable over time.

On the other hand, an instance represents the actual data within the database at any particular moment, which can change frequently as the database is updated.

What is Schema?

Schema is the overall description of the database. In DBMS, the term schema refers to the architecture of the database which describes how it will appear or will be constructed. It describes the organization of data such as tables, relationships as well as constraints. It also dictates how data items in a database will be stored, arranged, and accessed.

Schema is of three types: Logical Schema, Physical Schema and view Schema. 

• Logical Schema - It describes the database designed at a logical level.
• Physical Schema - It describes the database designed at the physical level.
• View Schema - It defines the design of the database at the view level.

Example:

Let's say a table teacher in our database named school, the teacher table requires the name, dob, and doj in their table so we design a structure as:  

Teacher table
name: String
doj: date
dob: date

Advantages of Schema

• Consistency: Guarantees proper storage of data in order to allow easy access and expandability.
• Structure: Helps in easy arrangement of the data base in an organized manner and hence makes it easy to comprehend.
• Data Integrity: Puts in place restrictions that ensure the data’s maintaining of its accuracy and subsequent reliability.

Disadvantages of Schema

• Rigidity: Schemas, defined, may be rigid for alteration, and may take a huge amount of effort in order to alter the scheme.
• Complexity: Developing a schema may be difficult or time consuming in case of large databases.

What is Instance?

An instance of DBMS refers to real data present in a database at some particular point in time. It refers to the content in the database in as much as it refers to the structure defined under a particular schema at a given point.

Example

Let's say a table teacher in our database whose name is School, suppose the table has 50 records so the instance of the database has 50 records for now and tomorrow we are going to add another fifty records so tomorrow the instance has a total of 100 records. This is called an instance. 

Advantages of Instance

• Real-Time Representation: It a return of the data in the database at a certain point in time as may be required for analysis or for performing operations.
• Flexibility: While a schema remains fixed in time, instances can be quite volatile, as data is written, updated, or deleted.

Disadvantages of Instance

• Volatility: Those are occurrences may be dynamic in a way they are different over time and this may make it a challenge to keep track without the necessary intervention.
• Data Integrity Issues: If not well regulated, it is evident that the data in an instance could become very inconsistent and at times even incorrect.

Difference Between Schema and Instance

Conclusion

In short, the schema is the blueprint of the database, while the instance is the actual data that is in the database. The schema is the database’s design, and the instance is the data it contains. When you change the schema, you change the structure of the database. When you change the instance, you change the data content.