matching empty column

[Srinivas Iyyer <srini_iyyer_bio(at)yahoo(dot)com>]

hello group:

I have a table that has 500,000 rows. 

 tar_id | mid   | seq_id | lscore
--------+-------+--------+--------
  32014 |       |  50473 |    814
  32015 |    67 |  50473 |    814
  32016 |   461 |  50473 |    814
  32017 |       |  52964 |    857
  32018 |   428 |  52964 |    822
  32019 |    98 |  51208 |    737
  32020 |   435 |  51208 |    737
  32021 |       |  51208 |    764
  32022 |    98 |  41066 |    759
  32023 |   431 |  41066 |    593
  32024 |   250 |  41066 |    659

The mid column is empty in certain places. how can I
search if there are empty spaces in mid. 

Thanks.


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