SQL Exercise: The number of matches each referee has managed
49. From the following tables, write a SQL query to count the number of matches managed by referees of each country. Return country name, number of matches.
Sample table: match_mast
match_no | play_stage | play_date | results | decided_by | goal_score | venue_id | referee_id | audence | plr_of_match | stop1_sec | stop2_sec
----------+------------+------------+---------+------------+------------+----------+------------+---------+--------------+-----------+-----------
1 | G | 2016-06-11 | WIN | N | 2-1 | 20008 | 70007 | 75113 | 160154 | 131 | 242
2 | G | 2016-06-11 | WIN | N | 0-1 | 20002 | 70012 | 33805 | 160476 | 61 | 182
3 | G | 2016-06-11 | WIN | N | 2-1 | 20001 | 70017 | 37831 | 160540 | 64 | 268
4 | G | 2016-06-12 | DRAW | N | 1-1 | 20005 | 70011 | 62343 | 160128 | 0 | 185
5 | G | 2016-06-12 | WIN | N | 0-1 | 20007 | 70006 | 43842 | 160084 | 125 | 325
6 | G | 2016-06-12 | WIN | N | 1-0 | 20006 | 70014 | 33742 | 160291 | 2 | 246
7 | G | 2016-06-13 | WIN | N | 2-0 | 20003 | 70002 | 43035 | 160176 | 89 | 188
8 | G | 2016-06-13 | WIN | N | 1-0 | 20010 | 70009 | 29400 | 160429 | 360 | 182
9 | G | 2016-06-13 | DRAW | N | 1-1 | 20008 | 70010 | 73419 | 160335 | 67 | 194
....
51 | F | 2016-07-11 | WIN | N | 1-0 | 20008 | 70005 | 75868 | 160307 | 161 | 181
Sample table: referee_mast
referee_id | referee_name | country_id
------------+-------------------------+------------
70001 | Damir Skomina | 1225
70002 | Martin Atkinson | 1206
70003 | Felix Brych | 1208
70004 | Cuneyt Cakir | 1222
70005 | Mark Clattenburg | 1206
70006 | Jonas Eriksson | 1220
70007 | Viktor Kassai | 1209
70008 | Bjorn Kuipers | 1226
70009 | Szymon Marciniak | 1213
.......
70018 | Clement Turpin | 1207
Sample table: soccer_country
country_id | country_abbr | country_name
------------+--------------+---------------------
1201 | ALB | Albania
1202 | AUT | Austria
1203 | BEL | Belgium
1204 | CRO | Croatia
1205 | CZE | Czech Republic
1206 | ENG | England
1207 | FRA | France
1208 | GER | Germany
1209 | HUN | Hungary
........
1229 | NOR | Norway
Sample Solution:
SQL Code:
-- Selecting the country name and the count of matches officiated by referees from each country
SELECT
country_name, -- Selecting the country name from the soccer_country table
COUNT(match_no) -- Counting the number of matches for each country
FROM
match_mast a -- Specifying the match_mast table with alias 'a'
JOIN
referee_mast c ON a.referee_id = c.referee_id -- Joining the match_mast table with the referee_mast table based on referee ID
JOIN
soccer_country b ON c.country_id = b.country_id -- Joining the referee_mast table with the soccer_country table based on country ID
GROUP BY
country_name -- Grouping the results by country name
-- Ordering the results by the count of matches in descending order
ORDER BY
COUNT(match_no) DESC;
Sample Output:
country_name | count ----------------+------- England | 7 Italy | 4 Slovenia | 4 Spain | 3 Serbia | 3 Netherlands | 3 Turkey | 3 Germany | 3 Poland | 3 Sweden | 3 Hungary | 3 France | 2 Czech Republic | 2 Norway | 2 Russia | 2 Romania | 2 Scotland | 2 (17 rows)
Code Explanation:
The said query in SQL that selects the name of countries and the count of match numbers in the match_mast table for each country.
The JOIN clause in this query then joins the match_mast and referee_mast tables based on the referee_id column and joins the results with soccer_country tables based on the country_id column.
The query groups the results by country name using the GROUP BY clause and applies an aggregate function, count(), to count the number of match numbers for each country. The results are then ordered in descending order of the count of match numbers using the ORDER BY clause.
Alternative Solutions:
Subquery:
-- Selecting the country name and the count of matches officiated by referees from each country
SELECT
country_name, -- Selecting the country name from the subquery result
COUNT(match_no) -- Counting the number of matches for each country
FROM
( -- Subquery to join tables and retrieve match numbers and country names
SELECT
a.match_no, -- Selecting the match number from the match_mast table
b.country_name -- Selecting the country name from the soccer_country table
FROM
match_mast a -- Specifying the match_mast table with alias 'a'
JOIN
referee_mast c ON a.referee_id = c.referee_id -- Joining the match_mast table with the referee_mast table based on referee ID
JOIN
soccer_country b ON c.country_id = b.country_id -- Joining the referee_mast table with the soccer_country table based on country ID
) subquery -- Alias for the subquery
GROUP BY
country_name -- Grouping the results by country name
-- Ordering the results by the count of matches in descending order
ORDER BY
COUNT(match_no) DESC;
Explanation:
This query uses a subquery to first join the necessary tables and select the relevant columns. Then, it performs the grouping and counting operation on the result of the subquery.
Using HAVING Clause:
-- Selecting the country name and the count of matches officiated by referees from each country
SELECT
b.country_name, -- Selecting the country name from the soccer_country table
COUNT(a.match_no) -- Counting the number of matches for each country
FROM
match_mast a -- Specifying the match_mast table with alias 'a'
JOIN
referee_mast c ON a.referee_id = c.referee_id -- Joining the match_mast table with the referee_mast table based on referee ID
JOIN
soccer_country b ON c.country_id = b.country_id -- Joining the referee_mast table with the soccer_country table based on country ID
GROUP BY
b.country_name -- Grouping the results by country name
HAVING
COUNT(a.match_no) > 0 -- Filtering out countries with zero matches
-- Ordering the results by the count of matches in descending order
ORDER BY
COUNT(a.match_no) DESC;
Explanation:
This query directly groups the results by country_name and then uses the HAVING clause to filter out groups with zero matches. It then orders the result by the count of matches in descending order.
Cross Join and Grouping:
-- Selecting the country name and the count of matches officiated by referees from each country
SELECT
b.country_name, -- Selecting the country name from the soccer_country table
COUNT(a.match_no) -- Counting the number of matches for each country
FROM
match_mast a -- Specifying the match_mast table with alias 'a'
JOIN
referee_mast c ON a.referee_id = c.referee_id -- Joining the match_mast table with the referee_mast table based on referee ID
JOIN
soccer_country b ON c.country_id = b.country_id -- Joining the referee_mast table with the soccer_country table based on country ID
JOIN
(SELECT DISTINCT country_id FROM referee_mast) d ON b.country_id = d.country_id -- Joining with a subquery to ensure only countries with referees are included
GROUP BY
b.country_name -- Grouping the results by country name
-- Ordering the results by the count of matches in descending order
ORDER BY
COUNT(a.match_no) DESC;
Explanation:
This query introduces a cross join with a subquery that selects distinct country_id from referee_mast. This ensures that all countries are included in the result set. The rest of the query proceeds with grouping and counting as before.
Relational Algebra Expression:
Relational Algebra Tree:
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Practice Online
Sample Database: soccer
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