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# Note the missing salary.
julia> employee_csv = """
                  name,department,position,salary
                  "ANTHONY A","POLICE","POLICE OFFICER",72510
                  "DANIEL A","FIRE","FIRE FIGHTER-EMT",95484
                  "JAMES A","FIRE","FIRE ENGINEER-EMT",103350
                  "JEFFERY A","POLICE","SERGEANT",101442
                  "NANCY A","POLICE","POLICE OFFICER",80016
                  "ROBERT K","FIRE","FIRE FIGHTER-EMT",
                  """ |> IOBuffer |> CSV.File
6-element CSV.File{false}:
 CSV.Row: (name = "ANTHONY A", department = "POLICE", position = "POLICE OFFICER", salary = 72510)
 CSV.Row: (name = "DANIEL A", department = "FIRE", position = "FIRE FIGHTER-EMT", salary = 95484)
 CSV.Row: (name = "JAMES A", department = "FIRE", position = "FIRE ENGINEER-EMT", salary = 103350)
 CSV.Row: (name = "JEFFERY A", department = "POLICE", position = "SERGEANT", salary = 101442)
 CSV.Row: (name = "NANCY A", department = "POLICE", position = "POLICE OFFICER", salary = 80016)
 CSV.Row: (name = "ROBERT K", department = "FIRE", position = "FIRE FIGHTER-EMT", salary = missing)

julia> filter(x->x.salary < 100_000, employee_csv)
ERROR: TypeError: non-boolean (Missing) used in boolean context
Stacktrace:
 [1] filter(f::var"#11#12", a::CSV.File{false})
   @ Base ./array.jl:2522
 [2] top-level scope
   @ REPL[29]:1

julia> filter(x-> coalesce(x.salary < 100_000, false), employee_csv)
3-element Vector{CSV.Row}:
 CSV.Row: (name = "ANTHONY A", department = "POLICE", position = "POLICE OFFICER", salary = 72510)
 CSV.Row: (name = "DANIEL A", department = "FIRE", position = "FIRE FIGHTER-EMT", salary = 95484)
 CSV.Row: (name = "NANCY A", department = "POLICE", position = "POLICE OFFICER", salary = 80016)

julia> chicago = DataKnot(:employee => employee_csv);

julia> @query chicago begin
        employee
        filter(salary < 100000)
        end
  │ employee                                        │
  │ name       department  position          salary │
──┼─────────────────────────────────────────────────┼
1 │ ANTHONY A  POLICE      POLICE OFFICER     72510 │
2 │ DANIEL A   FIRE        FIRE FIGHTER-EMT   95484 │
3 │ NANCY A    POLICE      POLICE OFFICER     80016 │