Casting to Float 2 dp in Postgres

Try the following, you are missing round() around average_salary and total_salary as you are just having , 2 without using round(). Also if you convert SUM(j.salary) to float then it should give you the expected result.

SELECT 
  j.job_title,
  round(SUM(j.salary)::float/COUNT(p.id), 2) AS average_salary,
  COUNT(p.id) AS total_people,
  round(SUM(j.salary)::float, 2) AS total_salary
FROM job j
    JOIN people p
    ON p.id = j.people_id
GROUP BY j.job_title
ORDER BY total_salary DESC
LIMIT 100

You can use a floating point, but you can't set it to a certain number of decimal places. The floating point types available in PostgreSQL are real or double precision.

You can either do this:

expression::real

or, to get 2 decimal places, use numeric(10,2), where 10 is the precision (the total number of digits) and 2 is the scale (the number of decimal places):

postgres=# SELECT (15::decimal/7)::decimal(10,2);
 numeric 
---------
    2.14
(1 row)

Note that you will have to cast one of the values in the expression as decimal, otherwise it will treat both as int, and the division will result in an integer, so casting to a decimal after that won't be accurate.

For example:

postgres=# SELECT (15/7)::decimal(10,2);
 numeric 
---------
    2.00
(1 row)

This also applies to the real type, where you would have to cast at least one of the values in your expression as real to get an accurate result.

See the documentation for more information: https://www.postgresql.org/docs/12/datatype-numeric.html

Try this one:

SELECT j.job_title,
  ROUND (AVG(j.salary), 2) ::FLOAT average_salary,
  COUNT(p.id) total_people ,
  ROUND (SUM(j.salary), 2) ::FLOAT total_salary
FROM people p
LEFT OUTER JOIN job j
ON (p.id = j.people_id)
GROUP BY j.job_title
ORDER BY average_salary DESC