SQL Exercise: Join within the tables salesman, customer and orders
SQL JOINS: Exercise-7 with Solution
Write a SQL statement to join the tables salesman, customer and orders so that the same column of each table appears once and only the relational rows are returned.
Sample table: orders
Sample table: customer
Sample table : salesman
Sample Solution:
SELECT *
FROM orders
NATURAL JOIN customer
NATURAL JOIN salesman;
Output of the Query:
salesman_id city customer_id ord_no purch_amt ord_date cust_name grade name commission 5005 London 3001 70009 270.65 2012-09-10 Brad Guzan Pit Alex 0.11 5001 New York 3002 70002 65.26 2012-10-05 Nick Rimando 100 James Hoog 0.15 5001 New York 3007 70005 2400.60 2012-07-27 Brad Davis 200 James Hoog 0.15 5001 New York 3002 70008 5760.00 2012-09-10 Nick Rimando 100 James Hoog 0.15 5006 Paris 3004 70010 1983.43 2012-10-10 Fabian Johnson 300 Mc Lyon 0.14 5001 New York 3002 70013 3045.60 2012-04-25 Nick Rimando 100 James Hoog 0.15
Explanation:
The said SQL query that uses the NATURAL JOIN clause to combine rows from three tables: orders, customer, and salesman.
The NATURAL JOIN clause compares all columns of the two joined tables and only returns the rows where the values match.
The query retrieves all columns from the three tables where the customer_id from the orders table matches the customer_id from the customer table, and the salesman_id from the orders table matches the salesman_id from the salesman table.
Note that using NATURAL JOIN can be dangerous if the tables have columns with the same name but different meanings, because it could lead to unexpected results.
Relational Algebra Expression:

Relational Algebra Tree:

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Previous SQL Exercise: Display commission of the salesman for an order.
Next SQL Exercise: Customer who works either through a salesman or by own.
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SQL: Tips of the Day
Grouped LIMIT in PostgreSQL: Show the first N rows for each group?
db=# SELECT * FROM xxx; id | section_id | name ----+------------+------ 1 | 1 | A 2 | 1 | B 3 | 1 | C 4 | 1 | D 5 | 2 | E 6 | 2 | F 7 | 3 | G 8 | 2 | H (8 rows)
I need the first 2 rows (ordered by name) for each section_id, i.e. a result similar to:
id | section_id | name ----+------------+------ 1 | 1 | A 2 | 1 | B 5 | 2 | E 6 | 2 | F 7 | 3 | G (5 rows)
PostgreSQL v9.3 you can do a lateral join
select distinct t_outer.section_id, t_top.id, t_top.name from t t_outer join lateral ( select * from t t_inner where t_inner.section_id = t_outer.section_id order by t_inner.name limit 2 ) t_top on true order by t_outer.section_id;
Database: PostgreSQL
Ref: https://bit.ly/3AfYwZI
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