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.
Существуют специальные операторы IS NULL и IS NOT NULL, которые позволяют производить сравнения с NULLами.
ID in both tables will never be NULL. It IS possible that one of the tables may have ID values that are NOT in the other table.
Предикат IS NULL позволяет проверить отсутствие (наличие) значения в полях таблицы. Использование в этих случаях обычных предикатов сравнения может привести к неверным результатам, так как сравнение со значением NULL дает результат UNKNOWN (неизвестно).
Each SELECT statement within the UNION ALL must have the same number of fields in the result sets with similar data types.
NULL is special in SQL. NULL indicates that the data is unknown, inapplicable or even does not
SELECT header, txt FROM news UNION ALL SELECT name, pass FROM members This will
R language supports several null-able values and it is relatively important to understand how these values behave, when making data pre-processing and