SQL Exercises: Display distinct salesman and their cities
SQL UNION: Exercise-2 with Solution
2. From the following tables, write a SQL query to find distinct salespeople and their cities. Return salesperson ID and city.
Sample table: Salesman
Sample table: Customer
Sample Solution:
SELECT salesman_id, city
FROM customer
UNION
(SELECT salesman_id, city
FROM salesman)
Sample Output:
salesman_id city 5001 New York 5002 London 5002 California 5006 Paris 5007 Rome 5002 Paris 5005 London 5003 Berlin 5007 Moscow 5003 San Jose
Code Explanation:
The said query in SQL that retrieves a list of unique pairs of salesman_id and city from two different tables, customer and salesman. The UNION operator is used to combine the results of two separate SELECT statements into a single result set.
The resulting table will contain all the unique combinations of salesman_id and city that appear in either the customer or salesman tables.
Relational Algebra Expression:

Relational Algebra Tree:

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Previous SQL Exercise: Display all salesmen and customer located in London.
Next SQL Exercise: Salesmen, customer involved in inventory management.
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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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