SQL Exercises: Salesmen, customer involved in inventory management
SQL UNION: Exercise-3 with Solution
3. From the following tables, write a SQL query to find all those salespeople and customers who are involved in the inventory management system. Return salesperson ID, customer ID.
Sample table: orders
Sample table: customer
Sample Solution:
SELECT salesman_id, customer_id
FROM customer
UNION
(SELECT salesman_id, customer_id
FROM orders)
Sample Output:
salesman_id customer_id 5005 3001 5007 3003 5001 3007 5002 3008 5006 3004 5003 3009 5001 3002 5002 3005
Code Explanation:
The said query in SQL which selects the distinct set of salesman IDs and customer IDs from two tables - customer and orders - and combines them using the UNION operator.
The UNION operator combines the results of two SELECT statements into a single table. It automatically removes any duplicate rows, so the resulting table will have a distinct set of salesman_id and customer_id pair.
Relational Algebra Expression:

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Previous SQL Exercise: Display distinct salesman and their cities.
Next SQL Exercise: Largest and smallest orders are produced on each date.
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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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