How to get non-group by column X value of the first row of every group by column Y of table T?

I have a table T with columns X, Y and Z. I need to retrieve non-group by column X value of the first row of ev开发者_StackOverflow中文版ery group, group by column Y value, and MIN of column Z value in a SQL single query.

Please could you help me out.

I've assumed that you've got a column x_dt that can be used to determine the first row of a Y group.

SELECT
  x,
  y,
  z
FROM (SELECT
  x,
  y,
  MIN(z) OVER (PARTITION BY y) AS z,
  ROW_NUMBER() OVER (PARTITION BY y ORDER BY x_dt) AS rn
FROM T) T2
WHERE rn = 1;