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SELECT * FROM A ,B WHERE CASE WHEN ISNULL(A.COL1,'')='' THEN A.B ELSE A.A END=B.A
select * from 表A as a ,表B as b where (case when a.列1 is null then b.列1 else b.列1 )= b.列1 and b.列2= a.列2
select isnull(a.列1,b.列1),a.列2
from tb1 a left join tb2 on a.列2=b.列2