SQL Exercise: Customer who works either through a salesman or by own

SQL JOINS: Exercise-8 with Solution

From the following tables write a SQL query to display the customer name, customer city, grade, salesman, salesman city. The results should be sorted by ascending customer_id.

Sample table: customer

 customer_id |   cust_name    |    city    | grade | salesman_id 
-------------+----------------+------------+-------+-------------
        3002 | Nick Rimando   | New York   |   100 |        5001
        3007 | Brad Davis     | New York   |   200 |        5001
        3005 | Graham Zusi    | California |   200 |        5002
        3008 | Julian Green   | London     |   300 |        5002
        3004 | Fabian Johnson | Paris      |   300 |        5006
        3009 | Geoff Cameron  | Berlin     |   100 |        5003
        3003 | Jozy Altidor   | Moscow     |   200 |        5007
        3001 | Brad Guzan     | London     |       |        5005

Sample table: salesman

 salesman_id |    name    |   city   | commission 
-------------+------------+----------+------------
        5001 | James Hoog | New York |       0.15
        5002 | Nail Knite | Paris    |       0.13
        5005 | Pit Alex   | London   |       0.11
        5006 | Mc Lyon    | Paris    |       0.14
        5007 | Paul Adam  | Rome     |       0.13
        5003 | Lauson Hen | San Jose |       0.12

Sample Solution:

-- Selecting specific columns from the 'customer' and 'salesman' tables
SELECT a.cust_name, a.city, a.grade, 
       b.name AS "Salesman", b.city 
-- Specifying the tables to retrieve data from ('customer' as 'a' and 'salesman' as 'b')
FROM customer a 
-- Performing a left join based on the salesman_id, including unmatched rows from 'customer'
LEFT JOIN salesman b 
ON a.salesman_id = b.salesman_id 
-- Sorting the result set by customer_id in ascending order
ORDER BY a.customer_id;

Output of the Query:

cust_name	city		grade	Salesman	city
Brad Guzan	London			Pit Alex	London
Nick Rimando	New York	100	James Hoog	New York
Jozy Altidor	Moscow		200	Paul Adam	Rome
Fabian Johnson	Paris		300	Mc Lyon		Paris
Graham Zusi	California	200	Nail Knite	Paris
Brad Davis	New York	200	James Hoog	New York
Julian Green	London		300	Nail Knite	Paris
Geoff Cameron	Berlin		100	Lauson Hen	San Jose

Explanation:

The said SQL query that uses the LEFT JOIN clause to combine rows from two tables: customer and salesman.
The query retrieves the cust_name, city, grade, name, and city from the two tables where the salesman_id from the customer table matches the salesman_id from the salesman table.
The query also uses the AS keyword to rename the name column as 'Salesman'.
The result of the query will show all rows from the customer table and the matching rows from the salesman table, and any customer without a matching salesman will have NULL values for the salesman columns.
The query also uses the ORDER BY clause to sort the result by customer_id.

Visual Explanation:

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Query Visualization:

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Previous SQL Exercise: Join within the tables salesman, customer and orders.
Next SQL Exercise: Customers who holds a grade less than 300.