A one-to-many
or many-to-one relationship between input data sets implies that one
data set has at most one observation with a specific value of the
selected variable, but the other input data set can have more than
one occurrence of each value. When you work with multiple selected
variables, this relationship implies that each combination of values
occurs no more than once in one data set. However, the combination
can occur more than once in the other data set. The order in which
the input data sets are processed determines whether the relationship
is one-to-many or many-to-one.
In the following example,
observations in data sets ONE and TWO are related by common values
for variable A. Values of A are unique in data set ONE but not in
data set TWO.
In the following example,
observations in data sets ONE, TWO, and THREE are related by common
values for variable ID. Values of ID are unique in data sets ONE and
THREE but not in TWO. For values 2 and 3 of ID, a one-to-many relationship
exists between observations in data sets ONE and TWO, and a many-to-one
relationship exists between observations in data sets TWO and THREE.