Question

Database is Oracle. I have a single table with three columns, A, B and C.

• A is a string column
• B is an int column
• C is an int column

I need an SQL select that groups on A and returns the share (in percent) where B = C = [particular int value] for each value of A.

Something like

SELECT A, percentBEqC
...
GROUP ON A

Answer 1

SELECT A
     , COUNT( CASE WHEN B = C AND C = 256
                   THEN 1
              END 
            )
       / COUNT(*) AS percentBEqC
FROM YourTable
GROUP BY A

You may want to check if B = 256 AND C = 256 has better performance.

---

Another way for your problem would be:

SELECT ta.A
     , COALESCE(cnt,0) / cntAll AS percentBEqC
FROM
    ( SELECT A
           , COUNT(*) AS cntAll
      FROM YourTable
      GROUP BY A
    ) ta
  LEFT JOIN
    ( SELECT A
           , COUNT(*) AS cnt
      FROM YourTable
      WHERE B = 256 AND C = 256
      GROUP BY A
    ) ts
    ON ts.A = ta.A