SQL Server Escape an Underscore

I had a similar issue using like pattern '%_%' did not work - as the question indicates :-)

Using '%\_%' did not work either as this first \ is interpreted "before the like".

Using '%\\_%' works. The \\ (double backslash) is first converted to single \ (backslash) and then used in the like pattern.

Obviously @Lasse solution is right, but there's another way to solve your problem: T-SQL operator LIKE defines the optional ESCAPE clause, that lets you declare a character which will escape the next character into the pattern.

For your case, the following WHERE clauses are equivalent:

WHERE username LIKE '%[_]d';            -- @Lasse solution
WHERE username LIKE '%$_d' ESCAPE '$';
WHERE username LIKE '%^_d' ESCAPE '^';

T-SQL Reference for LIKE:

You can use the wildcard pattern matching characters as literal characters. To use a wildcard character as a literal character, enclose the wildcard character in brackets. The following table shows several examples of using the LIKE keyword and the [ ] wildcard characters.

For your case:

... LIKE '%[_]d'