# Field as Variable in Query.jl
**URL:** https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365
**Category:** Data
**Created:** [February 27, 2018, 1:55pm UTC](https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365 "2018-02-27T13:55:09Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@RobertR](https://discourse.julialang.org/u/RobertR)
#### Post date: [February 27, 2018, 1:55pm UTC](https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365/1 "2018-02-27T13:55:09Z")
I’m a bit confused about why something (admittedly stupid) I was trying to do didn’t work.
Using Query.jl you can write something like:
```julia
x = @from i in df begin
@where i.age>50
@select {i.name, i.children}
@collect DataFrame
end
```
Which goes through a DataFrame `df`, finds all rows where `df[:age]` is greater than 50 and returns `df[:name]` and `df[:children]`. I’d like to use these macros with variables for the data frame columns.
My idea was to do something like `eval(parse("i.$variable"))`. That way I could create a function taking in `variable`, which then gets read into the string, which goes through parse and eval and (in my mind) should just act as `i.variable` in the `@where` macro.
Doing this with some random type works fine and returns the field `variable` of that type, if it exists. But in the `@where` macro it says that `i` is not defined. I assume this happens because of the scope of the `@where` macro, but I clearly don’t understand how macros work well enough to know why it doesn’t work.
Could somebody help me out a bit? For context, my final goal is to make simple way to go through a large-ish amount of data given a couple of selection criteria, so I’m trying to make a function which could take in a text input and parse parts of it as Query.jl query statements.
So effectively I’d like to make a function that takes in a string like `"age>50, name children"` and puts `i.age>50` into `@where` and `i.name, i.children` into `@select`.
---
### Author:  [@piever](https://discourse.julialang.org/u/piever)
#### Post date: [February 27, 2018, 2:44pm UTC](https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365/2 "2018-02-27T14:44:14Z")
You can try `@where getfield(i, variable) > 50` (you could even put this in a function `f(i)`). I’m not sure how to do for the `select` statement though.
---
### Author:  [@davidanthoff](https://discourse.julialang.org/u/davidanthoff)
#### Post date: [February 28, 2018, 5:59am UTC](https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365/3 "2018-02-28T05:59:43Z")
I played around with this a bit and didn’t get it to work, in the end I ran into world age problems…
This is what I tried:
```julia
function foo(name)
f = eval(parse("i->@NT($name = i.$name)"))
@from i in df begin
@select f(i)
@collect
end
end
```
Not sure whether there is a solution to this…
@RobertR, could you post a bit more of the code that you tried? Just wondering whether that might be a more promising avenue.
---
### Author:  [@RobertR](https://discourse.julialang.org/u/RobertR)
#### Post date: [February 28, 2018, 12:50pm UTC](https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365/4 "2018-02-28T12:50:18Z")
Thanks for the help, managed to come up with a solution:
```julia
function simple_query(q_where::Symbol, q_comp::Symbol, q_val::Any, q_select::Symbol, df::DataFrame)
@from i in df begin
@where eval(Expr(:call, q_comp, getfield(i, q_where), q_val))
@select eval(getfield(i, q_select))
end
end
```
For example, the inputs would be:
`simple_query(:age, :(>), 50, :name, i)`
My attempt to bodge together a way to pass these through as a string:
```julia
function string_query(string_in, df)
query_where, query_select = split(string_in, ", ")
comp_operators = ["==", ">", ">=", "<", "<="]
comp = ""
for c in comp_operators
if contains(query_where, c)
comp = c
end
end
query_where_field, query_where_val = split(query_where, string(" ", comp, " "))
if parse(query_where_val) isa Number
q_val = parse(query_where_val)
else
q_val = replace(String(query_where_val), "\"", "")
end
return simple_query(Symbol(query_where_field), Symbol(comp), q_val, Symbol(query_select), df)
end
```
Into which you can just pass `string_query("age > 50, name", df)` and it should work.
There’s some weird bit with parsing non-numericals (which I assume are strings) there as my data uses string ID’s like `"60001158002"` which were being read as Int and causing problems. To query them I decided to make it so that `string_query("obsid == \"60001158002\", Downloaded", df)` reads `"\"60001158002\""` as a SubString, then converts it to a String and strips the `"`'s so it ends up calling `simple_query(:obsid, :(==), "60001158002", :Downloaded, df)`.
Not very pretty but it works.
---
### Author:  [@davidanthoff](https://discourse.julialang.org/u/davidanthoff)
#### Post date: [March 1, 2018, 5:09am UTC](https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365/5 "2018-03-01T05:09:05Z")
Interesting, and glad it works. It is probably pretty slow, though?
---
### Author:  [@RobertR](https://discourse.julialang.org/u/RobertR)
#### Post date: [March 1, 2018, 9:28am UTC](https://discourse.julialang.org/t/field-as-variable-in-query-jl/9365/6 "2018-03-01T09:28:54Z")
Oh yeah, way slower but not noticeably so for a user. With the same query on the same data for both using Query normally takes ~5.7ms and using `simple_query` with symbols takes ~168ms. This should only be used as a more ‘user friendly’ way to find a handful of results though so it shouldn’t be a problem.