您可以使用两个子查询：
SELECT a.*
, (SELECT Count(b.id) FROM inquiries I1 WHERE I1.dealer_id = a.id) as counttotal
, (SELECT SUM(b.cost) FROM inquiries I2 WHERE I2.dealer_id = a.id) as turnover
FROM dealers a
ORDER BY name ASC
或者
SELECT a.*
, COALESCE(T.counttotal, 0) as counttotal -- use coalesce or equiv. to turn NULLs to 0
, COALESCE(T.turnover, 0) as turnover -- use coalesce or equiv. to turn NULLs to 0
FROM dealers a
LEFT OUTER JOIN (SELECT a.id, Count(b.id) as counttotal, SUM(b.cost) as turnover
FROM dealers a1
INNER JOIN inquiries b ON a1.id = b.dealer_id
GROUP BY a.id) T
ON a.id = T.id
ORDER BY a.name
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