【SQL】SELECT col, COUNT(DISTINCT other_col) FROM tab GROUP BY col的两种写法

To count distinct values inside of a GROUP BY, use the following.

SELECT col, COUNT(DISTINCT other_col) FROM tab GROUP BY col

The same result can be achieved without using the DISTINCT keyword, as below.

SELECT col, COUNT(*) FROM
  (SELECT col, other_col FROM tab GROUP BY col, other_col) t
GROUP BY col