Update a column using Subquery in oracle

Question

I have an EMPLOYEE table below:

EMP_ID     DEPT_ID
101        1
102        2
103        3
104        1

And a DEPARTMENT table as:

DEPT_ID  COUNTS
1   
2   
3   

I want to write a query which would count the number of Employee that belong to a department and store it into Department column table so the Department table will look like:

DEPT_ID  COUNTS
1         2
2         1
3         1

The solution is

update department p
    set counts = (select count(*) from EMPLOYEE e where p.dept_id = e.dept_id);

But i really dont understand how it works internally How does it know which dept ids in DEPARTMENT it has to set counts to. what exactly does this subquery return "select count(*) from EMPLOYEE e where p.dept_id = e.dept_id"

Answer 1

This is called correlated subquery:

This is your query:

update department p
set counts = (select count(*) from EMPLOYEE e where p.dept_id = e.dept_id); 

So, the inner query get executed based on the outer query (ie has department table). If, your outer query has total 3 records then, inner query (ie has EMPLOYEE table) will get execute three times.

In your scenario, you have mapped the queries with dept_id from outer query, so this will look up into inner query (ie EMPLOYEE table) and get the counts based on dept_id .