select a,b,null,null from table1 union select null,null,c,d from table2 union select null,null,null,null,e,f from table3.
Union select null, null, null, null, null, null, null from information_schema.tables. for a small database containing three tables. this instruction is used in sql injection I tried it and it worked but I didn't really know how it works can somebody help me...
The Background I was trying to solve the fourth realistic mission in hackthissite.org, and couldn't figure out exactly what SQL I should inject into the URL to retrieve the list of emails.
Даже выражение NULL != NULL не будет истинным, ведь нельзя однозначно сравнить одну неизвестность с другой. Кстати, ложным это выражение тоже не будет, потому что при вычислении условий Oracle не ограничивается состояниями ИСТИНА и ЛОЖЬ.
Note: A NULL value is different from a zero value or a field that contains spaces. A field with a NULL value
E.g. null + null + 1 = 1 null + null + null = null. The problem is that the first expression yields null.
The Oracle IS NOT NULL condition is used to test for a NOT NULL value.
SQL’s NULL indicates absent data. NULL propagates through expressions and needs distinct comparison operators.
when I compare null values the instruccion fails.
In R language, NULL … Continue reading R null values: NULL, NA, NaN, Inf.