Data storage in base table and extended table

[Vipin Verma3]

When a table is extended, it inherits all the columns from the table it is extending. When we store data in extended table, records are created in base table as well. My question is on the data stored for columns that are in the base table. The data is only stored in the base table, or it is stored both in base table and extended table? 

If data is stored in both tables base and extended, then what is the reason of keeping the data in both tables?

[Bert_c1]

Try:

TaskTableFlattening

 and

table-extension-and-classes

Use the Search on the docs website.

[HéctorM85291199]

Thank you, very helpful!

[WillieW]

Click the Helpful button on posts you find helpful. Nice to do for others.

[FerhanB]

Thanks!

[anchaljha24]

When you work with extended tables, the system doesn’t actually duplicate the same data in both places.

Instead:

Base table columns → Their values are stored only in the base table.
Extended table columns → Their values are stored only in the extended table.
The two tables are linked by the same primary key (often a one-to-one relationship).

So when you query the extended table, the system performs a join behind the scenes: it pulls the base table’s data for inherited columns and the extended table’s data for new columns, presenting them as if they were a single record.

From a developer’s perspective, the extended table appears to have all the base columns.

But physically, those columns are not duplicated — they’re just referenced from the base table.

EXAMPLE : 

Base Table: Employee

- EmployeeID

- Name

- HireDate

Extended Table: Manager

- EmployeeID (same key)

- Department

 When you query Manager, you’ll see EmployeeID, Name, HireDate, Department.

But under the hood:

- Name and HireDate live in Employee.

- Department lives in Manager.

No duplication — just a logical merge at query time.