# Joining on a DataFrame on an argument and column

**URL:** <https://discourse.julialang.org/t/joining-on-a-dataframe-on-an-argument-and-column/124613>\
**Category:** New to Julia\
**Tags:** question, dataframes\
**Created:** [January 9, 2025, 10:56pm UTC](https://discourse.julialang.org/t/joining-on-a-dataframe-on-an-argument-and-column/124613 "2025-01-09T22:56:11Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Snowy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/snowy/32/36765_2.png) [@Snowy](https://discourse.julialang.org/u/Snowy)\
**Post date:** [January 9, 2025, 10:56pm UTC](https://discourse.julialang.org/t/joining-on-a-dataframe-on-an-argument-and-column/124613/1 "2025-01-09T22:56:11Z")

</div>

Hello,  
Is it possible to join DataFrames on an argument and a column header? E.g.:

```julia
New_DF = innerjoin(DF1,DF2, on = ID => :Store_ID)

```

where “ID” is an argument of a function and “:Store\_ID” is a specific column within DF2?

For more context, I have user input data that contains a DataFrame and I do not know the column name of the ID / Key. There is another DataFrame created in a separate function that I will eventually need to join with the user input data. Because the second DataFrame is created it will always have a consistent column header and thus I know what key to use.

I would expect this to work:

```julia

function join_the_df(;data_1::DataFrame,key::String)
    DF2 = DataFrame(ID=[20, 40], Job=["Lawyer", "Doctor"])
    New_DF = innerjoin(data_1,DF2, on = key => :ID)
    return(New_DF)
end

join_the_df(;
    data_1 = User_DF,
    key = "Identification"
    )

```

but it throws the error:  
`ERROR: TypeError: in keyword argument on, expected Union(AbstractVector, var"#s493"} where var"#s493"<:Union{AbstractString, Pair{Symbol, Symbol}, Symbol, Tuple{Symbol, Symbol}, Tuple{Symbol, Symbol}, Pair{<:AbstractString, <:AbstractString}}, got a value of type Pair{String, Symbol}`

(btw how do I copy out of the Terminal in VSCode so I don’t have to type this error message? 🙂 )

If I modify the code and hard-code the value in, it works… but this would defeat the purpose of the function. E.g.:

```User_DF

function join_the_df(;data_1::DataFrame)
    DF2 = DataFrame(ID=[20, 40], Job=["Lawyer", "Doctor"])
    New_DF = innerjoin(data_1,DF2, on = :Identification => :ID)
    return(New_DF)
end

join_the_df(;
    data_1 = User_DF,
    )

```

Thank you for your help!

---

<div class="post-metadata">

**Author:** ![Dan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dan/32/42581_2.png) [@Dan](https://discourse.julialang.org/u/Dan)\
**Post date:** [January 10, 2025, 4:28am UTC](https://discourse.julialang.org/t/joining-on-a-dataframe-on-an-argument-and-column/124613/2 "2025-01-10T04:28:30Z")

</div>

> [@Snowy](#):
>
> ```julia
> New_DF = innerjoin(DF1,DF2, on = ID => :Store_ID)
> 
> ```

Have you tried:

```julia
New_DF = innerjoin(DF1,DF2, on = Symbol(ID) => :Store_ID)

```

The `:somename` notation is a Julia _literal_ of type Symbol which can also be written `Symbol("somename")`.

---

<div class="post-metadata">

**Author:** ![Snowy](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/snowy/32/36765_2.png) [@Snowy](https://discourse.julialang.org/u/Snowy)\
**Post date:** [January 10, 2025, 3:26pm UTC](https://discourse.julialang.org/t/joining-on-a-dataframe-on-an-argument-and-column/124613/3 "2025-01-10T15:26:16Z")

</div>

Thanks – This worked. Would advocate for a bit better error message here though to help noobs like me 🙂 Earlier in my script I mixed arguments and literals together and DataFrames gave me a message saying, “did you mean to use Cols?” (The error message came from this issue: [why `mixing strings with other selectors is not allowed`? · Issue #3241 · JuliaData/DataFrames.jl · GitHub](https://github.com/JuliaData/DataFrames.jl/issues/3241)) would be useful to have something similar here.

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<div class="post-metadata">

**Author:** ![rocco\_sprmnt21](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rocco_sprmnt21/32/20127_2.png) [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)\
**Post date:** [January 11, 2025, 9:33pm UTC](https://discourse.julialang.org/t/joining-on-a-dataframe-on-an-argument-and-column/124613/4 "2025-01-11T21:33:07Z")

</div>

As the error message indicates, the key column identifiers for the join must be provided as **symbol pairs** or **string pairs**. A mixed pair is not accepted.

```julia
julia> using DataFrames

julia> function join_the_df(;data_1::DataFrame,key::String)
           DF2 = DataFrame(ID=[20, 40], Job=["Lawyer", "Doctor"])
           New_DF = innerjoin(data_1,DF2, on = key => "ID")
           return(New_DF)
       end
join_the_df (generic function with 1 method)

julia> User_DF = DataFrame(Identification=[10, 20, 30, 40], name=["Ab", "Bc", "Cd", "De"])
4×2 DataFrame
 Row │ Identification name   
     │ Int64 String
─────┼────────────────────────
   1 │ 10 Ab
   2 │ 20 Bc
   3 │ 30 Cd
   4 │ 40 De

julia> join_the_df(;
           data_1 = User_DF,
           key = "Identification"
           )
2×3 DataFrame
 Row │ Identification name Job    
     │ Int64 String String
─────┼────────────────────────────────
   1 │ 20 Bc Lawyer
   2 │ 40 De Doctor

```