[mysql] SQL WHERE condition is not equal to?

Is it possible to negate a where clause?

e.g.

DELETE * FROM table WHERE id != 2;

This question is related to mysql sql where-clause negate

The answer is


Yes. If memory serves me, that should work. Our you could use:

DELETE FROM table WHERE id <> 2

Use <> to negate the where clause.


Best solution is to use

DELETE FROM table WHERE id NOT IN ( 2 )

You could do the following:

DELETE * FROM table WHERE NOT(id = 2);

I was just solving this problem. If you use <> or is not in on a variable, that is null, it will result in false. So instead of <> 1, you must check it like this:

 AND (isdelete is NULL or isdelete = 0)

delete from table where id <> 2



edit: to correct syntax for MySQL


Your question was already answered by the other posters, I'd just like to point out that

 delete from table where id <> 2

(or variants thereof, not id = 2 etc) will not delete rows where id is NULL.

If you also want to delete rows with id = NULL:

delete from table where id <> 2 or id is NULL

WHERE id <> 2 should work fine...Is that what you are after?


Look back to formal logic and algebra. An expression like

A & B & (D | E)

may be negated in a couple of ways:

  • The obvious way:

    !( A & B & ( D | E ) )
    
  • The above can also be restated, you just need to remember some properties of logical expressions:

    • !( A & B ) is the equivalent of (!A | !B).
    • !( A | B ) is the equivalent of (!A & !B).
    • !( !A ) is the equivalent of (A).

    Distribute the NOT (!) across the entire expression to which it applies, inverting operators and eliminating double negatives as you go along:

        !A | !B | ( !D & !E )
    

So, in general, any where clause may be negated according to the above rules. The negation of this

select *
from foo
where      test-1
  and      test-2
  and (    test-3
        OR test-4
      )

is

select *
from foo
where NOT(          test-1
           and      test-2
           and (    test-3
                 OR test-4
               )
         )

or

select *
from foo
where        not test-1
  OR         not test-2
  OR   (     not test-3
         and not test-4
       )

Which is better? That's a very context-sensitive question. Only you can decide that.

Be aware, though, that the use of NOT can affect what the optimizer can or can't do. You might get a less than optimal query plan.


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