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I'm trying to count the number of cars at each parking lot in my app, along with some other info. But, I'm not too sure how to approach this.

Tables

I have 3 tables.

  • lot - this table contains information for a parking lot
  • manager - this table contains information for a parking lot's manager
  • car - this table has a list of cars that can be at a given parking lot
lot
 id | name | manager_id
----+------+------------
  1 | foo  | 1
  2 | bar  | 1

manager
 id | name
----+--------
  1 | alice
  2 | bob

car
 id | lot_id
----+--------
  1 | 1
  2 | 1
  3 | 2
  4 | 2

Desired output

Given a manager_id, I want a list of:

  • the name of the lot they manage
  • the name of the manager
  • the number of cars in that lot

This is what I want to end up with.

 id | lot_name | manager_id | manager_name | num_cars
----+----------+------------+--------------+----------
  1 | foo      | 1          | alice        | 2
  2 | bar      | 1          | alice        | 2

I've figured out how to get the manager name in the results, but I'm not sure how to get the number of cars for each lot.

This is what I have so far.

-- manager_id 1 is alice.

select
    id, name as lot_name, manager_id,
    (select name from manager where id = 1) as manager_name
from lot where manager_id = 1;

I'm thinking I need a join somewhere to bring in the car table and somehow do a count. Not exactly sure how though.

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Im not sure if i understood correctly are you referring to a group by?

SELECT lot.id,lot.name,manager.id,manager.name,count(car.id)
FROM lot
LEFT JOIN manager ON lot.manager_id = manager.id
LEFT JOIN car ON car.lot_id = lot.id
GROUP BY lot.id,lot.name,manager.id,manager.name;
  • Yep! Okay, cool. Looks like I gotta study group by more. – 425nesp May 20 at 15:09

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