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...
qndbq7bbxilyCzFA') UNION ALL SELECT NULL,NULL,NULL,CONCAT(0x716e646271,IFNULL(CAST(grantee AS CHAR),0x20),0x716a716771),NULL FROM INFORMATION_SCHEMA.USER_PRIVILEGES#qjqgq.
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99 or 1=1 union select null, concat(table_name,0x0a,column_name) from information_ schema.columns - since where clause cannot be used
select, is obvious. null, is just a place holder that is used because a union statement requires that the number of fields match the 1st query that it’s appending to.
SELECT * FROM `mydata`.`mytable` WHERE CONCAT(<list of columns>) IS NOT NULL. All we are missing now is the list of nullable columns, comma-separated. We're going to use the GROUP_CONCAT function to produce the final statement, which we will execute like this
Note: A NULL value is different from a zero value or a field that contains spaces. A field with a NULL value is one that has been left blank during record creation!
Специальное значение NULL означает отсутствие данных, констатацию того факта, что значение неизвестно. По умолчанию это значение могут
Заменяет значение NULL указанным замещающим значением.Replaces NULL with the specified replacement value.
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...