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...
Note: A NULL value is different from a zero value or a field that contains spaces.
E.g. null + null + 1 = 1 null + null + null = null. The problem is that the first expression yields null.
NULL handling in the Oracle database is even more special. Contrary to common believe, using
The NOT NULL constraint specifies that NULL is not an allowable value. For example, in the following statement, CREATE TABLE Customer (SID integer NOT NULL
NULL is special in SQL. NULL indicates that the data is unknown, inapplicable or even does not exist.
SQL NOT NULL Constraint. By default, a column can hold NULL values. The NOT NULL constraint enforces a column to NOT accept NULL values. This enforces a field to always contain a value, which means that you cannot insert a new record, or update a record without adding a value to this field.
In R language, NULL … Continue reading R null values: NULL, NA, NaN, Inf.