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A񡜧
thread_id msg_box read
1 4 1
2 1 1
3 1 0
4 1 1

B񡜧
_id read
2 0
3 1
4 1
5 1
6 0

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select count(*)+(select count(*) from b where read=1)
from a
where read=1

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A񡜧
thread_id msg_box read
1 4 1
2 1 1
3 1 0
4 1 1

B񡜧
_id read
2 0
3 1
4 1
5 1
6 0

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select count(*)+(select count(*) from b where read=1)
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ÒýÓÃÂ¥Ö÷ furlxy µÄ»Ø¸´:
A񡜧
thread_id msg_box read
1 4 1
2 1 1
3 1 0
4 1 1

B񡜧
_id read
2 0
3 1
4 1
5 1
6 0

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ÎÒµÄSQLÓï¾äÊÇÕâÑù£º
select count(*)+(select count(*) from b¡­¡­

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SQL code
select
count(1)
from
(
select read from a
union all
select read from b
)t

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