SQL有条件地应用求和平均值

Question

假设我们有下表：

index  value
1      55  
2      66  
3      77  
1      88  
3      99

在一个select语句中，如何在index = 1或index = 3时求和（值），在index = 2时求平均值（avg）？

Answer 1

容易：

SELECT index, CASE WHEN index IN (1, 3) THEN SUM(value) ELSE AVG(value) END
FROM yourTable
GROUP BY index

Answer 2

SELECT sum(value) as agg,
       'S' as aggregateType
FROM DaTAble
WHERE index IN (1, 3)

UNION ALL

SELECT avg(value) as agg,
       'A' as aggregateType
FROM DaTAble
WHERE index = 2