Pipe the results of an expression into another. Like the bash operator.
Given a sample.yml file of:
a:b: cat
then
yq eval '.a | .b' sample.yml
will output
cat
Given a sample.yml file of:
a: cowb: sheepc: same
then
yq eval '.a = "cat" | .b = "dog"' sample.yml
will output
a: catb: dogc: same