Question

I have this table :

select count(distinct clean_deep_link) from tbl_1;
+---------------------------------+
| count(distinct clean_deep_link) |
+---------------------------------+
|                          121211 |
+---------------------------------+

I have this query :

select count(1) from tbl_1 where clean_deep_link IN
  (select clean_deep_link from tbl_2);

+----------+
| count(1) |
+----------+
|    66360 |
+----------+

But when I change the query to not in it returns an empty set :

select count(1) from tbl_1
where clean_deep_link not in (select clean_deep_link from tbl_2);
+----------+
| count(1) |
+----------+
|        0 |
+----------+

How is this possible? if the subquery contains about half of the records, shouldn’t the not of the subquery contain the other half? What am I missing here?

Thanks

Answer 1

I would assume that tbl_1.clean_deep_link is NULL for the rest of the rows.

These values are neither IN nor NOT IN your sub-query.

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The other reason could be, that you have NULL in tbl_2.clean_deep_link.

Please try the following:

select count(1) from tbl_1
where clean_deep_link not in (select clean_deep_link
                              from tbl_2 WHERE clean_deep_link IS NOT NULL);

The problem with NULL is that it is neither =, nor <> any other value (including NULL).

When checking for NOT IN, MySQL needs to check for each value in tbl_1 that it is not contained in tbl_2 and thus checks if they are <>.

Your values were not <> NULL, so they were not NOT IN.

See also: Using NOT IN operator with null values

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Check example in SQL Fiddle.