$years=Serial::find()->from(['new_table'=>Serial::find()->orderBy(['year'=>SORT_DESC])])->select('year')->groupBy('year')->orderBy(['year'=>SORT_DESC])->all();
SELECT `year`
FROM (
SELECT *
FROM `fl_serial`
ORDER BY `year` DESC) `new_table`
GROUP BY `year`
SELECT `year`
FROM (
SELECT *
FROM `fl_serial`
) `new_table`
GROUP BY `year`
ORDER BY `year` DESC
SELECT DISTINCT(year) FROM fl_serial ORDER BY `year` DESC
Serial::find()->select('year')->orderBy(['year'=>SORT_DESC])->distinct();