update user score from other tables - php

Question

I have a few tables that each have their own scores for each user. I would like to create a trigger that will add up all those scores for each user and put them in a field called score in the users table.
Tables (They essentially have the same fields with a few different ones) :
Table 1 : {id, user_id, score}
Table 2 : {id, user_id, score}
Table 3 : {id, user_id, score}
users : {id, name, overall_score}
// Overall _score has a value already , so i just want to add the score fields from the other tables to this one.

Answer 1

To achieve this lets first write the select query and get sum of all the scores per user from the 3 given tables and this is how it could be done
select u.*, y.total from users u
left join
(
select user_id,sum(score) as total from(
select user_id, score from table_1
union all
select user_id, score from table_2
union all
select user_id, score from table_3
)x group by x.user_id
)y on y.user_id = u.id
Here is the demo http://www.sqlfiddle.com/#!9/6f936/1
Now lets convert the select to an update command and it will be as
update users u
left join
(
select user_id,sum(score) as total from(
select user_id, score from table_1
union all
select user_id, score from table_2
union all
select user_id, score from table_3
)x group by x.user_id
)y on y.user_id = u.id
set u.overall_score = coalesce(y.total,0)
here is the demo http://www.sqlfiddle.com/#!9/c6993/1

Answer 2

To select data from multiple tables, you can use SQL JOINS.
See the example below:
SELECT table1.score, table2.score, table3.score
FROM table1 LEFT JOIN table2
ON table1.id=table2.id LEFT JOIN table3 ON table1.id=table3.id
This code will select the score column from table1, table2, and table3 and create one row per user_id, each containing one score-column/ table (in this case 3/ row). It's almost like having a fourth table containing all the scores, and then when you fetch them in PHP, it'd be like fetching an existing row from the database.
EDIT:
To Update the users table in the same query, you could use something like this:
UPDATE `users`,
(
SELECT table1.id as tid, table1.score as t1,
table2.score as t2, table3.score as t3
FROM table1 LEFT JOIN table2 ON table1.id=table2.id
LEFT JOIN table3 ON table1.id=table3.id
) as total
SET total_score = (t1 + t2 + t3) WHERE id = tid