Add token type '@'.

dbms/src/Parsers/Lexer.cpp

@@ -304,6 +304,8 @@ Token Lexer::nextTokenImpl()
                 return Token(TokenType::Concatenation, token_begin, ++pos);
             return Token(TokenType::ErrorSinglePipeMark, token_begin, pos);
         }
+        case '@':
+            return Token(TokenType::At, token_begin, ++pos);
 
         default:
             if (isWordCharASCII(*pos))

dbms/src/Parsers/Lexer.h

@@ -48,6 +48,8 @@ namespace DB
     M(GreaterOrEquals) \
     M(Concatenation)          /** String concatenation operator: || */ \
     \
+    M(At)                     /** @. Used only for specifying user names. */ \
+    \
     /** Order is important. EndOfStream goes after all usual tokens, and special error tokens goes after EndOfStream. */ \
     \
     M(EndOfStream) \