MySQL SUM（列）+（SUM的子查询）

Question

I want to make a query that adds up points for a team. The points are added up by doing SUM on a column + a SUM from another table with the same team-id. I try to write it like this:

SELECT 
    k.id, 
    s.fylke, 
    s.Kommune, 
    s.Skolenavn, 
    k.schoolid, 
    k.number, 
    k.letter, 
    SUM(e.amount) + (SELECT SUM(poeng) FROM oppdrag WHERE klasseid=k.id ) AS poeng 
FROM skoler AS s, klasser AS k, etappe AS e 
WHERE s.id=k.schoolid AND k.id=e.klasseid AND e.year='2016'
GROUP BY k.id 
ORDER BY poeng

The problem is that, when I write it in this fashion, it gives correct amount of points if the team has an entry in table "oppdrag" otherways it just returns NULL as points (poeng).

Answer 1

If your subquery returns NULL , this will result in trying to sum a number with NULL , which gives NULL back.

To fix this, you can try using the IFNULL function, that will replace it with a 0 if the query doesn't return any values:

SELECT 
    k.id, 
    s.fylke, 
    s.Kommune, 
    s.Skolenavn, 
    k.schoolid, 
    k.number, 
    k.letter, 
    SUM(e.amount) + IFNULL((SELECT SUM(poeng) FROM oppdrag WHERE klasseid=k.id ), 0) AS poeng 
FROM skoler AS s, klasser AS k, etappe AS e 
WHERE s.id=k.schoolid AND k.id=e.klasseid AND e.year='2016'
GROUP BY k.id 
ORDER BY poeng

Answer 2

I'd avoid that subquery from your select statement, it's going to kill your performance. Also, you should really be using the correct join syntax (the style you're using is ancient). Try something like this;

SELECT 
    k.id, 
    s.fylke, 
    s.Kommune, 
    s.Skolenavn, 
    k.schoolid, 
    k.number, 
    k.letter, 
    SUM(e.amount) + ISNULL(SUM(od.poeng),0) AS poeng 
FROM skoler AS s
INNER JOIN klasser AS k 
    ON s.id = k.schoolid
INNER JOIN etappe AS e 
    ON k.id = e.klasseid
LEFT JOIN oppdrag od 
    ON od.klasseid = k.id
WHERE e.year='2016'
GROUP BY k.id 
ORDER BY poeng

Answer 3

First, learn to use explicit JOIN syntax. Simple rule: Never use commas in the FROM clause. Always use explicit JOIN with ON .

Then, you can solve your problem by including the subquery in the FROM clause:

SELECT ske.*, ske.amount + COALESCE(od.amount, 0) as poenb
FROM (SELECT k.id, s.fylke, s.Kommune, s.Skolenavn, k.schoolid, 
             k.number, k.letter, SUM(e.amount) as amount
      FROM skoler s JOIN
           klasser k
           ON s.id = k.schoolid JOIN
           etappe e 
           ON k.id = e.klasseid AND e.year = '2016' 
      GROUP BY k.id 
     ) ske LEFT JOIN
     (SELECT od.klasseid, SUM(od.poeng) as amount
      FROM oppdrag od
      GROUP BY od.klasseid
     ) od
     ON od.klasseid = k.id
ORDER BY poeng;

Answer 4

Use explicit JOIN syntax. Also, include all columns that aren't aggregated in GROUP BY clause. Just because MySQL doesn't yield an error it doesn't mean that it's advisable to do it the way you did it. Not including all columns that aren't aggregated in a group by means picking random values.

SELECT 
    k.id, 
    s.fylke, 
    s.Kommune, 
    s.Skolenavn, 
    k.schoolid, 
    k.number, 
    k.letter, 
    COALESCE(SUM(e.amount), 0) + COALESCE(SUM(o.poeng), 0) AS poeng 
FROM skoles AS s
INNER JOIN klasser AS k ON s.id = k.schoolid
INNER JOIN etappe AS e ON k.id = e.klasseid
LEFT JOIN ( SELECT klasseid, SUM(poeng) AS poeng FROM oppdrag ) AS o ON o.klasseid = k.id
WHERE e.year = '2016'
GROUP BY k.id, s.fylke, s.Kommune, s.Skolenavn, k.schoolid, k.number, k.letter
ORDER BY poeng