Use LIMIT on two tables with an Inner Join

Question

The following SQL code works fine. It has the purpose of displaying all the products ordered by each user and sorting them by products in a descending manner.

The problems occur when I try to limit the display of customers and their related products, so when i use LIMIT 2. I tried to insert LIMIT 2 immediately after the ORDER BY line, but there is a problem: I only display one customer with its 2 purchases, so in total I only have 2 lines printed.

cursor.execute('''SELECT Client.client_name, Purchases.product 
                  FROM Client
                  INNER JOIN Purchases 
                  ON Client.client_name = Purchases.client_name
                  ORDER BY client_name;''')

I would like to print 2 lines for each customer. Where and how should I enter LIMIT 2? Can you show me please? (trying not to write a too complicated sql, because i understand little of Sql and i would like to try to keep the code as simple as possible)

In the case of the example below, I would like to print James, George, Sophie and get this output:

client_name | product   |
------------+-----------+
James      | Hard Disk |
James      | Mouse     |
George     | Book      |
George     | Keyboard  |  
Sophie     | Mouse     |
Sophie     | Coffee    |

The two main tables I’m using are these:

CLIENT


client_name |  born  |
-----------+---------+
James      |   1988  |
George     |   1988  |
Sophie     |   1988  |

PURCHASES

client_name | product   |
------------+-----------+
James      | Hard Disk |
James      | Mouse     |
George     | Book      |
George     | Keyboard  |  
Sophie     | Mouse     |
Sophie     | Coffee    |
Harry      | Cd        |
Harry      | Book      |
Lily       | Mouse     |
Lily       | Desk      |

Asked By: Rob92

||

Source

Answer 1

You can use the row_number window function to count assign a number to each product per client, and then have a condition on it:

SELECT   client_name, product
FROM     (SELECT     Client.client_name, 
                     Purchases.product,
                     ROW_NUMBER() OVER (PARTITION BY Client.client_name 
                                        ORDER BY Purchases.product DESC) AS rn
          FROM       Client
          INNER JOIN Purchases  ON Client.client_name = Purchases.client_name) t
WHERE    rn <= 2
ORDER BY client_name ASC

Answered By: Mureinik

Use LIMIT on two tables with an Inner Join

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