Expr allows you to inject raw SQL segments into a \Dewdrop\Db\Select or
other context in which Dewdrop might otherwise quote and therefore
misinterpret the code.
$expression : string
Storage for the SQL expression.
Instantiate an expression, which is just a string stored as
an instance member variable.
The string containing a SQL expression.
__toString() : string
Return expression property when this object is used as a string (e.g.
concatenated with another string value).
The string of the SQL expression stored in this object.