Check Rows BEING inserted without running the query again

Hi,

How can I check/compare the rows selected for insertion while running the SQL like:

INSERT INTO A SELECT * FROM B.

So I can decide whether to cancel the script or keep the query running.

I can run statements like:

IF (SELECT COUNT(*) FROM B)=100

INSERT INTO A SELECT * FROM B

But it will take the time as query is running again.

Please support.

JB