Joining Data with Stata
Stata offers several commands for joining datasets.
Full Join
If the left and right datasets may overlap, use -merge- to ensure that duplicates are not created.
use "left.dta" merge 1:1 KEYVARS using "right.dta"
If they are known to not overlap, consider using -append- instead.
use "cohort1.dta" append using "cohort2.dta"
See also -frlink-.
Left, Right, or Inner Joins
Use -merge- as above and select cases based on the automatically created _merge variable.
use "left.dta" merge 1:1 KEYVARS using "right.dta" keep if _merge==1 | _merge==3 // left join keep if _merge==2 | _merge==3 // right join keep if _merge==3 // inner join