How to average count column but by specific occupations

user_17747142 0
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Result from current query

I'm currently doing an assignment for a SQL class and can't seem to figure out how to go on from this. Going on from what I have I can't seem to average the counts per occupation. With what I have so far I see how many books each person borrowed. If I try to average the column it just averages everything together when I need it specifically by occupation. Also, I Grouped by clientId because each client has multiple books borrowed.

The assignment is The average number of borrowed books by job title.

Borrower Table:

BorrowId(PK), ClientId(FK), BookId, BorrowDate

Client Table:

ClientId(PK), ClientFirstName, ClientLastName, ClientDoB, Occupation

This is what I have so far.

SELECT **, count(*)
FROM borrower
INNER JOIN client on borrower.ClientId = client.ClientId
GROUP BY borrower.ClientId

Here is what I get from the query that I have so far. My goal is to average these counts by occupation. So, for example, I want it to show

Occupation Avg of occupation
Bus Driver 4 (14/4)
Cashier 5
Computer Engineer 3 (6/2)
Computer Programmer 5.666 (17/3)
count group-by mysql average sql
4 Answers

First aggregate only in the table Borrower to get the counts and then join to Client to get the average per Occupation:

SELECT c.Occupation, AVG(b.counter) average_count
FROM Client c
INNER JOIN (
  SELECT ClientId, COUNT(*) counter
  FROM Borrower
  GROUP BY ClientId
) b ON b.ClientId = c.ClientId
GROUP BY c.Occupation;
user_104988280

The solution is pretty simple:

SELECT Occupation, count(*) BooksBorrowed
FROM Borrower
NATURAL LEFT JOIN Clients
GROUP BY Occupation;

SQL fiddle

You just need to join tables and calculate count with group by occupation.

user_33993560

If you want to find the average number of borrowed books from all occupations, then use:

SELECT AVG(num_books)
FROM 
(
    SELECT COUNT(b.ClientId) AS num_books
    FROM client c
    LEFT JOIN borrower b ON b.ClientId = c.ClientId
    GROUP c.Occupation
) t;

The idea here is that each record in the intermediate subquery aliased as t above corresponds to the book count for a certain occupation. We then wrap that query and just take the average of all the counts to get the overall average book count.

user_18632290

INNER JOIN will help find the average count

SELECT Occupation as Jobtitle, count(*) as BooksBorrowed 
FROM Borrower 
NATURAL INNER JOIN Clients 
GROUP BY Occupation;
user_78463440
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