## In Stata, how do I merge two data sets in the many-to-one relationship?

Note: The `.merge` command supports 1:1, 1:m, m:1, and m:m match merges. For 1:1, see In Stata, how do I merge two data sets?

Suppose you have two data sets, `A.dta` and `B.dta` (see below), which share the same key variable, `id` :

Data set `A.dta` :

+---------+ | id y | |---------| | 1 85 | | 1 47 | | 2 95 | | 4 36 | | 5 83 | +---------+

Data set `B.dta` :

+---------+ | id x | |---------| | 1 53 | | 2 23 | | 3 45 | +---------+

To merge these two data sets, follow the appropriate instructions below.

### Stata 11 and later versions

Sort by key variable(s) first, and then enter the `merge` command, making sure the data set with the "many" observations is the current data set in memory (for m:1 merges). An example using the above data sets follows:

use "C:\temp\B.dta", clear sort id save "C:\temp\B.dta", replace use "C:\temp\A.dta", clear sort id merge m:1 id using "C:\temp\B.dta"

### Stata 10 and earlier versions

Use the `.joinby` command:

use "C:\temp\A.dta", clear sort id joinby id using "C:\temp\B.dta", unmatched(both)

### Results

In both versions, the `.merge` command creates a variable `_merge` indicating the results. For example, the designation `(3)` in the column on the right means that the observation appears in both data sets and that they're matched:

+--------------------------------+ | id y x _merge | |--------------------------------| | 1 85 53 matched (3) | | 1 47 53 matched (3) | | 2 95 23 matched (3) | | 4 36 . master only (1) | | 5 83 . master only (1) | | 3 . 45 using only (2) | +--------------------------------+

If you have questions about using statistical and mathematical software at Indiana University, email UITS Research Analytics (formerly known as the Stat/Math Center). Research Analytics is located on the IU Bloomington campus at Woodburn Hall 200, and is open for consultation by appointment Monday-Friday 9am-5pm. For more, visit Research Analytics on the web, or call 812-855-4724 (IUB) or 317-278-4740 (IUPUI).

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