select
count(case when coalesce(x.col1,1) = coalesce(y.col1,1) then null else 1 end) as cnt_col1,
count(case when coalesce(x.col2,"1") = coalesce(y.col2,"1") then null else 1 end) as cnt_col2
from
`DatasetA.Table` x
OUTER JOIN
`DatasetB.Table` y
on x.col1 = y.col1

col1, col2
null, null
null, null
1, null
null, 1

COLUMN, COUNT_OF_ERRORS
cnt_col1, 0
cnt_col2, 0
...
cnt_col15, 1