Нашёл 3 млн ответов for 'textbook advertising UNION ALL SELECT NULL,NULL-- RVnK'.

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UNION ALL SELECT NULL, *, NULL, NULL FROM email ... Use of an unqualified * with other items in the select list may produce a parse error. ... select NULL, email.*, NULL, NULL from email ... the tables when only using one table and selecting literals in addition to all the columns on that single...

All queries in an SQL statement containing a UNION operator must have an equal number of expressions in their target lists. Change the first part of the query to a null or negative value so we can see what field will echo data back to us.

Go Secure Coding Practices
select * from data where id=-1 union select null,null,database()

https://owasp.org/www-pdf-archive/Owasp-171123063052.pdf

242-31: SQL Set Operators: So Handy Venn You Need Them
Set operators are so designated because they are conceptually derived from mathematical set theory. The three basic set operators are UNION, INTERSECT, and EXCEPT. All three, with variations (in particular, OUTER UNION) and options, are implemented in PROC SQL.

https://support.sas.com/resources/papers/proceedings/proceedings/sugi31/242-31.pdf

SQL - unions clause
The SQL UNION clause/operator is used to combine the results of two or more SELECT statements without returning any duplicate rows. To use UNION, each SELECT must have the same number of columns selected, the same number of column expressions, the same data type...

http://www.tutorialspoint.com/sql/pdf/sql-unions-clause.pdf

Using the Set Operators | UNION ALL
When multiple SELECT queries are joined using UNION operator, Oracle displays the combined result from all the compounded SELECT queries,after removing all duplicates and in sorted order ascendingbydefault, without ignoring the NULL values.

https://www.tutorialspoint.com/sql_certificate/pdf/using_the_set_operators.pdf

Using the Set Operators Questions
A. UNION B. UNION ALL C. INTERSECT D. MINUS Answer: A. Set operators are used to combine the results of two ormore SELECT statements.Valid set A. It returns rows from the combined queries along with NULL values B. It returns rows for the combined queries after eliminating duplicates C. It...

https://www.tutorialspoint.com/sql_certificate/pdf/using_the_set_operators_questions.pdf

Microsoft Word - Blindfolded SQL Injection - Final.doc
Since performing a UNION SELECT statement requires knowledge of the number of fields in the query as well as the type of each field, it is often Once the NULL-based UNION statement works, it is a trivial process to identify the types of each column. In each iteration a single field is tested for its type.

https://www.imperva.com/docs/IMPERVA_HII_blind-sql-injection-attacks.pdf

Nulls Make Things Easier?
SELECT NULL < NULL + 1; ?column? -(null). NULL represents unknown, not applicable, or unassigned. It has no data type, so comparing it to xed values always returns NULL. WITH ordertest AS ( SELECT NULL UNION ALL SELECT 2 UNION ALL SELECT 1 UNION ALL SELECT NULL.

https://momjian.us/main/writings/pgsql/nulls.pdf

Java Security.key | ‘; "select * from employees where last_name = '';'"
INFORMATION_SCHEMA.CHARACTER_SETS GROUP BY x)a) AND 'hxTg'='hxTg. Type: UNION query Title: MySQL UNION query (NULL) - 6 columns Payload: name=Baaz' UNION ALL SELECT NULL,NULL,NULL,NULL,CONCAT(0x717a6b6471,0x4f6145586b4a6e436d71,0x7176646d71),NULL#.

https://s3.amazonaws.com/cmj-presentations/hacking-rwx-2017/Hacking_and_Hardening_Java_Web_Applications.pdf