Worst use of query

Query 1:

Wrong:
 

for(int i=0; i<query("SELECT COUNT ...."); i++)
{
    //code..
}

This query iterates n times til the loop ends. And result of the query doesn't change.

Correct:
 

count = query("SELECT COUNT ....");

for(int i=0; i<count; i++)
{
    //code..
}

Query 2:

DELETE FROM table;

This query contain no where clause.
Before execute the query delete or update just select and check the right row is affecting.

Query 3:
 

SELECT * FROM sometable 
WHERE field LIKE '%something' 
OR LIKE 'something%' 
OR LIKE '%something%';

This query contains 3 like condition. Third condition selects first and second condition selects.

Query 4:
 

select * from users where clue > 0;
0 results found.

It never returns any results, no matter where and why you run it.

Query 5:
 

select * from *

Very bad.

Query 6:
 

DROP DATABASE;
ROLLBACK;

Really very bad.