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.
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).
NULL is special in SQL. NULL indicates that the data is unknown, inapplicable or even does not exist. In other words, NULL represents that the data is missing in the database. For example, if an employee does not have any phone number, you can store it as an empty string.
Тем не менее, NULL, как NULL-значение, часто используется в SQL. Предположим, появился покупатель, которому еще не назначен продавец. Чтобы констатировать этот факт, нужно ввести значение NULL в поле snum, а реальное значение включить туда позже...
The SQL standard does not define NULL as a value but rather as a placeholder for a missing or unknown
R language supports several null-able values and it is relatively important to understand how these values behave, when making data pre-processing and data