Select operations on R data frames

The R language is weird - particularly for those coming from a typical programmer's background, which likely includes OO languages in the curly-brace family and relational databases using SQL. A key data structure in R, thedata.frame, is used something like a table in a relational database. In terms of R's somewhat byzantine type system (which is explained nicely here), a data.frame is a list of vectors of varying types. Each vector is a column in the data.frame making this a column-oriented data structure as opposed to the row-oriented nature of relational databases.
In spite of this difference, we often want to do the same so
rts of things to an R data.frame that we would to a SQL table. The R docs confuse the SQL-savvy by using different terminology, so here is a quick crib-sheet for applying SQL concepts to data.frames.
We're going to use a sample data.frame with the following configuration of columns, or schema, if you prefer: (sequence:factor, strand:factor, start:integer, end:integer, common_name:character, value:double) where the type character is a string and a factor is something like an enum. Well, more accurately, value is a vector of type double and so forth. Anyway, our example is motivated by annotation of genome sequences, but the techniques aren't particular to any type of data.