the structure of the table is as follows:
Table 1 is associated with Table An and Table B once, and the union, code is roughly as follows:
select ta.*
from t1 join ta on t1.id_a = ta.id
where t1.flag = 'a'
union all
select tb.*
from t1 join tb on t1.id_b = tb.id
where t1.flag = 'b'
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