SQL Exercises: Any salesman was matched to the city of any customer
SQL UNION : Exercise-7 with Solution
7. From the following tables, write a SQL query that appends strings to the selected fields, indicating whether the city of any salesperson is matched with the city of any customer. Return salesperson ID, name, city, MATCHED/NO MATCH.
Sample table: Salesman
Sample table: Customer
Sample Solution:
SELECT a.salesman_id, name, a.city, 'MATCHED'
FROM salesman a, customer b
WHERE a.city = b.city
UNION
(SELECT salesman_id, name, city, 'NO MATCH'
FROM salesman
WHERE NOT city = ANY
(SELECT city
FROM customer))
ORDER BY 2 DESC
Sample Output:
salesman_id name city ?column? 5005 Pit Alex London MATCHED 5007 Paul Adam Rome NO MATCH 5002 Nail Knite Paris MATCHED 5006 Mc Lyon Paris MATCHED 5003 Lauson Hen San Jose NO MATCH 5001 James Hoog New York MATCHED
Code Explanation:
The said query in that retrieves information about salesmen and their cities, and whether or not they have customers in the same city.
The result set includes salesmen who have customers in their city, as well as salesmen who do not have customers in any city. The result set is sorted by the salesmen's names in descending order.
The uses of JOIN clause between the salesman and customer tables retrieve all salesmen who have customers in the same city. The SELECT statement also adds a column with the value 'MATCHED' to indicate that these salesmen have a match with a customer.
The second part of the query uses a subquery to retrieve all salesmen who do not have customers in any city. The SELECT statement also adds a column with the value 'NO MATCH' to indicate that these salesmen do not have a match with a customer.
The UNION operator combines the results of the two queries into a single result set.
The ORDER BY clause sorts the result set by the second column in descending order i.e., by the salesmen's names.
Practice Online

Query Visualization:
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Previous SQL Exercise: Salesmen who do not have customers in their cities.
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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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