MySQL查询不适用于空列

Question

I'm doing a relatively simple mysql query:

         SELECT g.id FROM myTable g
            WHERE g.timestamp > '0' 
            AND g.userId = '1'
            AND g.foo != '34'
            ORDER BY g.id DESC LIMIT 0, 10

This query returns 0 rows, however if I remove the last condition, ie change it to:

         SELECT g.id FROM myTable g
            WHERE g.timestamp > '0' 
            AND g.userId = '1'
            ORDER BY g.id DESC LIMIT 0, 10

That returns 5-6 rows. This is very strange to me, since in all of those rows, the int column foo is set to NULL

What am I doing wrong?

Answer 1

Comparing something wilh NULL will result in unknown . That is why NULL values have to be compared with the IS operator.

Replace

AND g.repostVia != '34'

with

AND (g.repostVia != '34' or g.repostVia  is null)

or use the NULL-safe equality operator (thanks to eggyal):

AND NOT g.repostVia <=> '34'

Answer 2

The comparison g.repostVia != '34' is FALSE when g.repostVia is NULL because any comparison with NULL will allways be FALSE.

You should make sure that you have covered this case (variable is NULL ) in any comparison with NULL-able column.

You can use IS NULL operator:

SELECT g.id FROM myTable g
            WHERE g.timestamp > '0' 
            AND g.userId = '1'
            AND (g.repostVia, != '34' OR g.repostVia IS NULL)
            ORDER BY g.id DESC LIMIT 0, 10

or COALESCE function:

SELECT g.id FROM myTable g
            WHERE g.timestamp > '0' 
            AND g.userId = '1'
            AND COALESCE(g.repostVia, -1) != '34'
            ORDER BY g.id DESC LIMIT 0, 10

I myself prefer COALESCE, because it seems to me that it retains the original meaning of comparison better and as it allows more parameters it is also more flexible.

Answer 3

Maybe try <> instead of != .

0 values are tricky.

Answer 4

Doing the comparison using IS NULL is definitely valid. I want to offer that COALESCE() is often a bit more readable, if you have an invalid value that you can assign. For instance, if g.foo is always positive, then the following would work:

     SELECT g.id FROM myTable g
         WHERE g.timestamp > '0'
          AND g.userId = '1'
         AND coalesce(g.foo, -1) != '34'
         ORDER BY g.id DESC
     LIMIT 0, 10