来自两个联接表的mySQL SELECT，子查询中的数据总和

Question

I have a table "schLoc" like this

------------------------
| id | ... | ... | ... |
------------------------
| 1  | ... | ... | ... |
| 2  | ... | ... | ... |
| 3  | ... | ... | ... |
| 4  | ... | ... | ... |
| 5  | ... | ... | ... |
| .. | ... | ... | ... |
------------------------

and another table like this "schLocDett" like this where schLoc.id=schLocDett.idDoc

-------------------------------
| idDoc | qta | ... | merce   |
-------------------------------
| 1     |  1  | ... | fattLoc |
| 1     |  1  | ... | fattSrv | 
| 2     |  3  | ... | fattLoc | 
| 2     |  2  | ... | notcSrv | 
| 2     |  2  | ... | fattSrv | 
| 3     |  5  | ... | fattSrv | 
| 3     |  3  | ... | notcSrv | 
| 3     |  3  | ... | fattLoc | 
| 3     |  7  | ... | fattLoc | 
| 4     |  5  | ... | notcSrv | 
| 4     |  4  | ... | fattSrv | 
| 4     |  1  | ... | fattSrv | 
| 5     |  1  | ... | notcSrv | 
| ...   | ... | ... | ....... | 
-------------------------------

I would like to have the list complete of schLoc and for each id the sum of qta associated but only for merce=fattLoc. In case that don't exist the sum will be 0. This is the result I expect

---------------------------
| id | sumQta | ... | ... |
------------------------
| 1  | 1      | ... | ... |
| 2  | 3      | ... | ... |
| 3  | 10     | ... | ... |
| 4  | 0      | ... | ... |
| 5  | 0      | ... | ... |
| .. | ...    | ... | ... |
---------------------------

I try with this:

SELECT TOT.sumQta, TAB.*, 
FROM schLoc AS TAB, schLocDett AS DETT,
    (SELECT idDoc, SUM(qta) AS sumQta 
     FROM schLocDett 
     WHERE merce='fattLoc' 
     GROUP BY idDoc) AS TOT
WHERE TAB.id>0 
  AND TAB.id=DETT.idDoc 
  AND TAB.id=TOT.idDoc

but not get Id 4 and 5

Any suggest are welcome

Answer 1

SELECT TAB.*, COALESCE(TOT.sumQta, 0) AS sumQta
FROM schLoc TAB
LEFT JOIN
(
    SELECT idDoc, SUM(qta) AS sumQta
    FROM schLocDett
    WHERE merce = 'fattLoc'
    GROUP BY idDoc
) TOT
    ON TAB.id = TOT.idDoc

Output:

Demo here:

Rextester

Answer 2

You can use LEFT JOIN with IFNULL , eg:

SELECT loc.id, SUM(IFNULL(det.qta, 0)) AS sum
FROM schLoc loc LEFT JOIN schLocDett det ON loc.id = det.idDoc
WHERE det.merce = 'fattLoc'
GROUP BY loc.id;