# Merging two dataframes of different length

**URL:** <https://discourse.julialang.org/t/merging-two-dataframes-of-different-length/79806>\
**Category:** New to Julia\
**Tags:** dataframes\
**Created:** [April 22, 2022, 1:20am UTC](https://discourse.julialang.org/t/merging-two-dataframes-of-different-length/79806 "2022-04-22T01:20:18Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Will\_G](https://avatars.discourse-cdn.com/v4/letter/w/f14d63/32.png) [@Will\_G](https://discourse.julialang.org/u/Will_G)\
**Post date:** [April 22, 2022, 1:20am UTC](https://discourse.julialang.org/t/merging-two-dataframes-of-different-length/79806/1 "2022-04-22T01:20:18Z")

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I am a beginner with Julia and am struggling to merge two dataframes I have of various length. This is one of the dataframes, then I have another that is 5120 long, in interest of space I have only included the first 20, but the second dataframe contains all the years the first dataframe has. I want to add G as a column to my second dataframe so that, every row with year=1965 has G=1.08906 and so on for each year. Any suggestions how I might do this - I apologize if there is an easy fix.

Row │ year G  
│ Float64 Float64  
─────┼──────────────────  
1 │ 1965.0 1.08906  
2 │ 1966.0 1.10632  
3 │ 1967.0 1.09697  
4 │ 1968.0 1.09504  
5 │ 1970.0 1.12902  
6 │ 1971.0 1.13436  
7 │ 1972.0 1.14328  
8 │ 1973.0 1.14644  
9 │ 1975.0 1.14837  
10 │ 1976.0 1.14905  
11 │ 1977.0 1.15022  
12 │ 1978.0 1.15693  
13 │ 1979.0 1.16652  
14 │ 1980.0 1.17443  
15 │ 1981.0 1.1808  
16 │ 1983.0 1.17581  
17 │ 1984.0 1.18052

5120×3 DataFrame  
Row │ year id s  
│ Int64 Int64 Float64  
──────┼────────────────────────────  
1 │ 1965 100751 0.000706819  
2 │ 1965 104151 0.000943925  
3 │ 1965 104179 0.000898235  
4 │ 1965 105330 0.000309796  
5 │ 1965 110097 6.78435e-5  
6 │ 1965 110404 4.6729e-5  
7 │ 1965 110635 0.00147144  
8 │ 1965 110644 0.000311526  
9 │ 1965 110653 3.46141e-6  
10 │ 1965 110662 0.00114019  
11 │ 1965 110671 0.000150917  
12 │ 1965 110699 5.57286e-5  
13 │ 1965 110705 0.000406715  
14 │ 1965 117636 4.77674e-5  
15 │ 1965 122612 0.000192108  
16 │ 1965 123961 0.00063586  
17 │ 1965 124885 7.85739e-5  
18 │ 1965 126775 6.47283e-5  
19 │ 1965 126818 0.000563171  
20 │ 1965 127060 0.000426791  
21 │ 1965 127741 0.000514019

---

<div class="post-metadata">

**Author:** ![icweaver](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/icweaver/32/45764_2.png) [@icweaver](https://discourse.julialang.org/u/icweaver)\
**Post date:** [April 22, 2022, 1:54am UTC](https://discourse.julialang.org/t/merging-two-dataframes-of-different-length/79806/2 "2022-04-22T01:54:00Z")

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Hi @Will_G, welcome! No need to apologize at all, I think it’s a great question that I would also like to know the answer to. Not sure if this is the most idiomatic way to go about this, but this sounds like a good job for [`leftjoin`](https://dataframes.juliadata.org/stable/lib/functions/#DataAPI.leftjoin):

```julia
using DataFrames

julia> df1 = DataFrame(
           year = 1965:1970,
           G = rand(6),
       )
6×2 DataFrame
 Row │ year G        
     │ Int64 Float64  
─────┼─────────────────
   1 │ 1965 0.181731
   2 │ 1966 0.762532
   3 │ 1967 0.415284
   4 │ 1968 0.216933
   5 │ 1969 0.374068
   6 │ 1970 0.424626

julia> df2 = DataFrame(
           year = repeat(1965:1970, inner=2),
           id = rand(10_000:13_000, 12),
           s = rand(12),
       )
12×3 DataFrame
 Row │ year id s        
     │ Int64 Int64 Float64  
─────┼────────────────────────
   1 │ 1965 11862 0.929915
   2 │ 1965 11758 0.466497
   3 │ 1966 11479 0.542719
   4 │ 1966 11924 0.978926
   5 │ 1967 12506 0.774413
   6 │ 1967 10473 0.153476
   7 │ 1968 12482 0.41511
   8 │ 1968 12222 0.303588
   9 │ 1969 12220 0.636519
  10 │ 1969 10899 0.408869
  11 │ 1970 11823 0.633692
  12 │ 1970 11411 0.92418

julia> leftjoin(df2, df1; on=:year)
12×4 DataFrame
 Row │ year id s G        
     │ Int64 Int64 Float64 Float64? 
─────┼──────────────────────────────────
   1 │ 1965 11862 0.929915 0.181731
   2 │ 1965 11758 0.466497 0.181731
   3 │ 1966 11479 0.542719 0.762532
   4 │ 1966 11924 0.978926 0.762532
   5 │ 1967 12506 0.774413 0.415284
   6 │ 1967 10473 0.153476 0.415284
   7 │ 1968 12482 0.41511 0.216933
   8 │ 1968 12222 0.303588 0.216933
   9 │ 1969 12220 0.636519 0.374068
  10 │ 1969 10899 0.408869 0.374068
  11 │ 1970 11823 0.633692 0.424626
  12 │ 1970 11411 0.92418 0.424626

```

And then if you wanted to edit `df2` in place, you could just replace `leftjoin` with `leftjoin!`

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<div class="post-metadata">

**Author:** ![Will\_G](https://avatars.discourse-cdn.com/v4/letter/w/f14d63/32.png) [@Will\_G](https://discourse.julialang.org/u/Will_G)\
**Post date:** [April 22, 2022, 2:02am UTC](https://discourse.julialang.org/t/merging-two-dataframes-of-different-length/79806/3 "2022-04-22T02:02:41Z")

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@icweaver Thank you that worked perfectly!