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1.
select count(t1.id)
from table1 t1
where t1.id = (select t2.id from table2 t2 where t2......);

2.
select count(t1.id)
from table1 t1,table2 t2
where t1.id = t2.id
and t2......;

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Èç¹û²»ÊÇcount(),¶øÊÇt1.ÆäËû×Ö¶ÎÄØ?»¹ÓÐÇø±ðÂð

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select count(t1.id)
from table1 t1
where exists(select 1 from table2 t2 where t2......
  and t2.id=t1.id);

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1.
select count(t1.id)
from table1 t1
where t1.id = (select t2.id from table2 t2 where t2......);

2.
select count(t1.id)
from table1 t1,table2 t2
where t1.id = t2.id
and t2......;

ÎÊÏÂ,Õâ2ÖÐд·¨¶¼¿ÉÒÔ´ïµ½»ã×ܱí1µÄÄ¿µÄ°É?
ÄÇÓÐÐÔÄÜÉϵÄÇø±ðÂð?
Èç¹û²»ÊÇcount(),¶øÊÇt1.ÆäËû×Ö¶ÎÄØ?»¹ÓÐÇø±ðÂð

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select count(t1.id)
from table1 t1
where t1.id in (select t2.id from table2 t2 where t2......);

2