SQL Exercise: Display the average price of items of each company
SQL JOINS: Exercise-23 with Solution
From the following tables write a SQL query to calculate the average price of items of each company. Return average value and company name.
Sample table: company_mast
Sample table: item_mast
SELECT AVG(pro_price), company_mast.com_name FROM item_mast INNER JOIN company_mast ON item_mast.pro_com= company_mast.com_id GROUP BY company_mast.com_name;
Output of the Query:
avg com_name 5000.0000000000000000 Samsung 650.0000000000000000 iBall 1475.0000000000000000 Epsion 500.0000000000000000 Frontech 250.0000000000000000 Zebronics 3200.0000000000000000 Asus
The said SQL query is selecting the average price of all products (AVG(pro_price)) and the company name (com_name) from the tables item_mast and company_mast, and grouping the results by the company name.
The query is using an INNER JOIN to combine the data from the two tables, linking the 'pro_com' column in item_mast to the 'com_id' column in company_mast.
Relational Algebra Expression:
Relational Algebra Tree:
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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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