Question

For an example suppose I have a table with two columns X and Y. Now I want to select from this table some rows with the following rules:

1. Define a “group” to be a group of rows that have same value of the column Y.
2. From a group, only one row is chosen.
3. In a group, if one of the rows has X=a where a is predefined, then that row is chosen from that group.
4. In a group, if no row has X=a, then any row can be chosen.

What are my options to do this in a single SQL query?

Answer 1

Try this:

SELECT DISTINCT
  t1.Y,
  COALESCE(a2.x, a1.x) 'x'
FROM Tablename t1
LEFT JOIN
(
    SELECT Y, MIN(x) x
    FROM Tablename
    GROUP BY Y
) a1 ON t1.Y = a1.Y
LEFT JOIN
(
    SELECT Y, MIN(x) x
    FROM Tablename
    WHERE x = 'a'
    GROUP BY Y
) a2 ON t1.Y = a2.Y;

SQL Fiddle Demo