SQL Exercises: Customers along with the salesmen who works for them
SQL Query on Multiple Tables: Exercise-2 with Solution
From the following tables, write a SQL query to locate all the customers and the salesperson who works for them. Return customer name, and salesperson name.
Sample table: customer
Sample table: salesman
Sample Solution:
SELECT customer.cust_name, salesman.name
FROM customer,salesman
WHERE salesman.salesman_id = customer.salesman_id;
Output of the query:
cust_name name Nick Rimando James Hoog Brad Davis James Hoog Graham Zusi Nail Knite Julian Green Nail Knite Fabian Johnson Mc Lyon Geoff Cameron Lauson Hen Jozy Altidor Paul Adam Brad Guzan Pit Alex
Code Explanation:
The given query in SQL that joins the 'customer' and 'salesman' tables based on the "salesman_id" column. The result set includes the customer name (cust_name) and salesman name (name). The WHERE clause specifies the join condition between the two tables, which is that the "salesman_id" column must be equal in both tables.
Relational Algebra Expression:

Relational Algebra Tree:

Explanation:

Visual presentation:

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Previous SQL Exercise: Customer and salesmen who lives in the same city.
Next SQL Exercise: Customer lives in a city other than the salesman's.
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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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