Question

I am beginner at SQL and I am trying to create a query.

I have these tables:

CREATE TABLE Hospital (
    hid INT PRIMARY KEY,
    name VARCHAR(127) UNIQUE,
    country VARCHAR(127),
    area INT
);
CREATE TABLE Doctor (
    ic INT PRIMARY KEY,
    name VARCHAR(127),
    date_of_birth INT,
);
CREATE TABLE Work (
    hid INT,
    ic INT,
    since INT,
    FOREIGN KEY (hid) REFERENCES Hospital (hid),
    FOREIGN KEY (ic) REFERENCES Doctor (ic),
    PRIMARY KEY (hid,ic)
 );

The query is: What is the average in each country of the number of doctors working in hospitals of that country (1st column: each country, 2nd column: average)? Thanks.

Answer 1

You first need to write a query that counts the doctors per hospital

select w.hid, count(w.ic)
from work w
group by w.hid;

Based on that query, you can retrieve the average number of doctors per country:

with doctor_count as (
  select w.hid, count(w.ic) as cnt
  from work w
  group by w.hid
)
select h.country, avg(dc.cnt)
from hospital h
  join doctor_count dc on h.hid = dc.hid
group by h.country;

If you have an old DBMS that does not support common table expressions the above can be rewritten as:

select h.country, avg(dc.cnt)
from hospital h
  join (
    select w.hid, count(w.ic) as cnt
    from work 
    group by w.hid
  ) dc on h.hid = dc.hid;

Here is an SQLFiddle demo: http://sqlfiddle.com/#!12/9ff79/1

Btw: storing date_of_birth as an integer is a bad choice. You should use a real DATE column.

And work is a reserved word in SQL. You shouldn’t use that for a table name.