The jq command syntax is something I always have to look up.
Command
aws rds describe-db-snapshots --db-snapshot-identifier rdspostgresqldb-snapshot-test20191130-1 \
--query "DBSnapshots[].{DBInstanceIdentifier:DBInstanceIdentifier,DBSnapshotIdentifier:DBSnapshotIdentifier,SnapshotCreateTime:SnapshotCreateTime}" \
--output json | \
jq -r ".[] | [.DBInstanceIdentifier,.DBSnapshotIdentifier,.SnapshotCreateTime] | @csv"
Result
"rdspostgresqldb","rdspostgresqldb-snapshot-test20191130-1","2019-11-30T06:47:14.370Z"