How does table extensions work?

[juan9]

I am trying to understand how servicenow table extension works, but I am stuck on how the data is distributed.

at first I thought when I extend another table, and I add a record to the child table that it would create a record in both parent and child tables, but thats not the case.

case study:

a table (child table) extends another (parent table).

I add a column(c) to the child table.

parent table has column a,b

child table has column c

when i create a new record in child table with this

a = "a"

b = "b"

c = "c"

How is that data stored in child table if it does not have columns a and b?

does it create two records, one in each table with the same sys_id?

how is the data split?

[juan9]

my best guess for understanding this is that it has two be two records with the same sys_id.
I created a dummy table that extended the task table and ran a query to see if there was a record with the same sys_id and it showed 

[juan9]

I created a record in the child table and it appears as it does create a record in both tables with the same sys_id

[Siddhesh Jadhav · Accepted solution]

Hi @juan9 ,

When a table in ServiceNow is extended, the child table inherits all fields (columns) from the parent table. However, the data is not stored in two separate records; rather, it is stored in a single record with the same sys_id in both the parent and child tables.

Here’s how the data is managed:

• The child table inherits the fields a and b from the parent table, so when you create a record in the child table, it will store the values for all fields (a, b, and c) in a single record.
• ServiceNow stores this record in the child table, but it links to the parent table by using the same sys_id.
• When you query the parent table, it will return data only from fields a and b. When querying the child table, it will return all fields, including a, b, and c.
• So, no two records are created. The data from the parent table is accessible to the child table, and both use the same sys_id to connect their data.

Thanks & Regards,

Siddhesh Jadhav

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