select from tables with different numbers of rows - mysql

Question

I'm hoping there is a simple answer to this. Competitors race over a series of 3 races. Some competitors only show up for one race. How could I show a final result for ALL competitors?
race 1
+------+--------+
| name | result |
+------+--------+
| Ali | 30 |
| Bob | 28 |
| Cal | 26 |
+------+--------+
race 2
+------+--------+
| name | result |
+------+--------+
| Ali | 32 |
| Bob | 31 |
| Dan | 24 |
+------+--------+
race 3
+------+--------+
| name | result |
+------+--------+
| Eva | 23 |
| Dan | 25 |
+------+--------+
The final result should look like this:
+------+--------+--------+--------+
| name | result | result | result |
+------+--------+--------+--------+
| Ali | 30 | 32 | |
| Bob | 28 | 31 | |
| Cal | 26 | | |
| Dan | | 24 | 25 |
| Eva | | | 23 |
+------+--------+--------+--------+
The problem I have is with ordering by name from multiple tables.
Here is the example data:
CREATE TABLE race (name varchar(20), result int);
CREATE TABLE race1 LIKE race;
INSERT INTO race1 VALUES ('Ali', '30'), ('Bob', '28'), ('Cal', '26');
CREATE TABLE race2 like race;
insert INTO race2 VALUES ('Ali', '32'), ('Bob', '31'), ('Dan', '24');
CREATE TABLE race3 LIKE race;
INSERT INTO race3 VALUES ('Eva', '23'), ('Dan', '25');
Many thanks!

Answer 1

Here we go !!!
select race1.name as name, race1.result, race2.result, race3.result from race1
left join race2 on race2.name = race1.name
left join race3 on race3.name = race1.name
union
select race2.name as name, race1.result, race2.result, race3.result from race2
left join race1 on race1.name = race2.name
left join race3 on race3.name = race2.name
union
select race3.name as name, race1.result, race2.result, race3.result from race3
left join race1 on race1.name = race3.name
left join race2 on race2.name = race3.name;
It is working :)

Answer 2

select s.name,
max(case when s.R = 'Result1' then s.result else '' end) as result1,
max(case when s.R = 'Result2' then s.result else '' end) as result2,
max(case when s.R = 'Result3' then s.result else '' end) as result3
from
(
select 'Result1' as R,r1.* from race1 r1
union all
select 'Result2' as R,r2.* from race2 r2
union all
select 'Result3' as R,r3.* from race3 r3
) s
group by s.name
result
+------+---------+---------+---------+
| name | result1 | result2 | result3 |
+------+---------+---------+---------+
| Ali | 30 | 32 | |
| Bob | 28 | 31 | |
| Cal | 26 | | |
| Dan | | 24 | 25 |
| Eva | | | 23 |
+------+---------+---------+---------+
5 rows in set (0.00 sec)

Answer 3

I personally would create the schema in a different way.
One table for the users, one for the races and one that connects both:
-- Create syntax for TABLE 'races'
CREATE TABLE `races` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- Create syntax for TABLE 'users'
CREATE TABLE `users` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- Create syntax for TABLE 'race_results'
CREATE TABLE `race_results` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`race_id` int(11) NOT NULL,
`user_id` int(11) NOT NULL,
`result` int(11) NOT NULL,
`created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
Let's insert some data (should be equal to your data set).
-- Insert data
INSERT INTO users (name)values('Ali'),('Bob'),('Cal'),('Dan'), ('Eva');
INSERT INTO races (name)values('Race1'),('Race2'),('Race3');
INSERT INTO race_results (user_id, race_id, result)values(1,1,30),(2,1,30),(1,2,28),(2,2,31),(3,1,26),(4,2,24),(4,3,25),(5,3,23);
Then you could write the query like this:
-- Static version
SELECT us.name, sum(if(ra.name='Race1', result, null)) as Race1, sum(if(ra.name='Race2', result, null)) as Race2, sum(if(ra.name='Race3', result, null)) as Race3
FROM race_results as rr
LEFT JOIN users as us on us.id = rr.user_id
LEFT JOIN races as ra on ra.id = rr.race_id
GROUP BY us.id;
Which gives you the result you're looking for. (I changed the column names to make it more obvious which result belongs to which race.)
But I've to admit that this works fine for 3 races but what if you have 30 or more?
Here is a more dynamic version of the above query, which kind of creates itself ;)
-- Dynamic version
SET #sql = '';
SELECT
#sql := CONCAT(#sql,if(#sql='','',', '),temp.output)
FROM
(SELECT
CONCAT("sum(if(ra.name='", race.name, "', result, null)) as ", race.name) as output
FROM races as race
) as temp;
SET #sql = CONCAT("SELECT us.name,", #sql, " FROM race_results as rr LEFT JOIN users as us on us.id = rr.user_id LEFT JOIN races as ra on ra.id = rr.race_id GROUP BY 1;");
SELECT #sql;
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;