Database Query Help required in MySQL - sql

Question

I am looking for help in writing a query of retrieving the values from 2 tables in MySQL.
The scenario is
Table A
ID Name Marks
===================
23 John 67
45 Mark 45
12 Ram 87
Table B has the following Structure
ID Name Evaluation Marks
==============================
45 Mark 34
78 Chris 09
98 Nancy 10
23 John 12
I am trying to write a query, where if I execute the following query
Select "SOMETHING" from Table A where Id=45
I should get Marks Column as 45+34=79, which should fetch and sum from the both the Tables A and Table B.
If I execute the query with the Id=12.
Since the Id=12, does not exists in the Table B, I should get the Marks as 87.
What would a query for the above?

Answer 1

I assume that the id occurs only once in your tables table a, but could be missing in both. If it always exists in table a, you can use a LEFT JOIN instead of the UNION.
SELECT COALESCE(SUM(marks), 0)
FROM
(
SELECT marks FROM a WHERE id = 45
UNION ALL
SELECT SUM(evaluation_marks) AS marks FROM b WHERE id = 45
) x
Edit
If you have all users in table a, then use
SELECT a.marks + COALESCE( SUM( b.evaluation_marks ), 0 )
FROM a
LEFT OUTER JOIN b ON ( b.id = a.id )
WHERE a.id = 45
GROUP BY a.id, a.marks
You should consider changing your table model though. Why do you store name and id twice? Can't you do it like that:
id name marks evaluation marks
=======================================
12 Ram 87 0
23 John 67 12
45 Mark 45 34
78 Chris 0 9
98 Nancy 0 10