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

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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?

To merge 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 | +---------+

Stata 11

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

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) | +--------------------------------+

For more about statistical and mathematical software, email the UITS Stat/Math Center, visit the center's web page, or phone 812-855-4724 (IUB) or 317-278-4740 (IUPUI). The center is located in Bloomington at 410 N. Park Avenue, and is open for consultation by appointment Monday-Friday 9am-5pm.

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Last modified on March 25, 2011.