How to dynamically change the data in the default value?

Question

PostgreSQL

There is a table of positions (1) in it there is a field number of employees (2). There is a table of employees (3). It is necessary that In the field number of employees (2) there is a default value of the number of employees (3) in this position (1) and that when adding / deleting a new / old employee (3) their number in the position table (3 ) changed.

Answer 1

You need triggers
https://en.wikipedia.org/wiki/%D0%A2%D1%80%D0%B8%D...