SQL Exercises: Display all salesmen and customer located in London
SQL UNION: Exercise-1 with Solution
1. From the following tables, write a SQL query to find all salespeople and customers located in the city of London.
Sample table: Salesman
Sample table: Customer
SELECT salesman_id "ID", name, 'Salesman' FROM salesman WHERE city='London' UNION (SELECT customer_id "ID", cust_name, 'Customer' FROM customer WHERE city='London')
ID name ?column? 3001 Brad Guzan Customer 3008 Julian Green Customer 5005 Pit Alex Salesman
The said query in SQL that retrieves the ID, name, and "Salesman" or "Customer" label for all salespeople and customers located in London. The results are combined using the UNION operator to create a single set of results.
The WHERE clause filters the results to only include rows where the city column equals 'London' for the salesman table.
The UNION operator combines the results of the first query with the results of a second query, which is enclosed in parentheses.
The WHERE clause in the second query filters the results to only include rows where the city column equals 'London' for the customer table.
The overall result set includes all rows from both queries with duplicates removed.
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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;
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