dbQuoteString() returns an object that can be coerced to character, of the same length as the input. For an empty character vector this function returns a length-0 object.

When passing the returned object again to dbQuoteString() as x argument, it is returned unchanged. Passing objects of class SQL should also return them unchanged. (For backends it may be most convenient to return SQL objects to achieve this behavior, but this is not required.)


The returned expression can be used in a SELECT ... query, and for any scalar character x the value of dbGetQuery(paste0("SELECT ", dbQuoteString(x)))[[1]] must be identical to x, even if x contains spaces, tabs, quotes (single or double), backticks, or newlines (in any combination) or is itself the result of a dbQuoteString() call coerced back to character (even repeatedly). If x is NA, the result must merely satisfy is.na(). The strings "NA" or "NULL" are not treated specially.

NA should be translated to an unquoted SQL NULL, so that the query SELECT * FROM (SELECT 1) a WHERE ... IS NULL returns one row.

Passing a numeric, integer, logical, or raw vector, or a list for the x argument raises an error.