_ (Wildcard - Match One Character) (Transact-SQL)
Matches any single character in a string comparison operation that involves pattern matching, such as LIKE and PATINDEX. For more information, see Pattern Matching in Search Conditions.
Examples
The following example uses the _ operator to find all the people in the Person table, who have a three-letter first name that ends in an.
USE AdventureWorks2008R2;
GO
SELECT FirstName, LastName
FROM Person.Person
WHERE FirstName LIKE '_an'
ORDER BY FirstName;