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select a.ID,Count(b.A1ID)--指定列
from A1 AS a
LEFT OUTER JOIN A2 AS b
on a.ID=b.A1ID
group by a.ID
SELECT ID,b.A2Count
FROM A1,
(
SELECT A1ID,COUNT(*) as A2Count
FROM A2
GROUP BY A1ID
)b
WHERE A1.ID=b.A1ID
--若表A1上的ID有索引会更快.
--加索引
create index A1_id on A1(id)
create index A2_a1id on A2(A1id)
select ID,Count(*)
from A1 AS a
LEFT OUTER JOIN A2 AS b
on a.ID=b.A1ID
group by ID