SQL Exercise: Salesmen work for one or more customers or yet to join
SQL JOINS: Exercise-14 with Solution
Write a SQL statement to make a list for the salesmen who either work for one or more customers or yet to join any of the customer. The customer, may have placed, either one or more orders on or above order amount 2000 and must have a grade, or he may not have placed any order to the associated supplier.
Sample table: customer
Sample table: salesman
Sample table: orders
Sample Solution:
SELECT a.cust_name,a.city,a.grade,
b.name AS "Salesman",
c.ord_no, c.ord_date, c.purch_amt
FROM customer a
RIGHT OUTER JOIN salesman b
ON b.salesman_id=a.salesman_id
LEFT OUTER JOIN orders c
ON c.customer_id=a.customer_id
WHERE c.purch_amt>=2000
AND a.grade IS NOT NULL;
Output of the Query:
cust_name |city |grade|Salesman |ord_no|ord_date |purch_amt| --------------|----------|-----|----------|------|----------|---------| Nick Rimando |New York | 100|James Hoog| 70002|2012-10-05| 65.26| Geoff Cameron |Berlin | 100|Lauson Hen| 70004|2012-08-17| 110.50| Brad Davis |New York | 200|James Hoog| 70005|2012-07-27| 2400.60| Nick Rimando |New York | 100|James Hoog| 70008|2012-09-10| 5760.00| Fabian Johnson|Paris | 300|Mc Lyon | 70010|2012-10-10| 1983.43| Geoff Cameron |Berlin | 100|Lauson Hen| 70003|2012-10-10| 2480.40| Jozy Altidor |Moscow | 200|Paul Adam | 70011|2012-08-17| 75.29| Nick Rimando |New York | 100|James Hoog| 70013|2012-04-25| 3045.60| Graham Zusi |California| 200|Nail Knite| 70001|2012-10-05| 150.50| Graham Zusi |California| 200|Nail Knite| 70007|2012-09-10| 948.50| Julian Green |London | 300|Nail Knite| 70012|2012-06-27| 250.45|
Explanation:
The said SQL query that is joining three tables: customer alias a, salesman alias b, and orders alias c. The query is using a RIGHT OUTER JOIN on salesman and customer tables on the 'salesman_id' column, and a LEFT OUTER JOIN on orders and customer tables on the 'customer_id' column.
The query is then selecting several columns from each table: 'cust_name', 'city', 'grade' from 'customer' table, 'name' aliased as 'Salesman' from salesman table, 'ord_no', 'ord_date', 'purch_amt' from orders table.
The query is also filtering the results based on the condition "c.purch_amt >=2000" and "a.grade IS NOT NULL" which means it will only return the rows where the purchase amount is greater than or equal to 2000 and grade is not null.
Visual Explanation:


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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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