优化聚合查询

Question

我正在寻找一种方法来优化以下内容：

SELECT 
    (SELECT SUM(amount) FROM Txn_Log WHERE gid=@gid AND txnType IN (3, 20)) AS pendingAmount,
    (SELECT COUNT(1) FROM Txn_Log WHERE gid = @gid AND txnType = 11) AS pendingReturn,
    (SELECT COUNT(1) FROM Txn_Log WHERE gid = @gid AND txnType = 5) AS pendingBlock

其中@gid是参数，gid是此表上的索引字段。问题:每个子查询在同一组条目上重新运行-三次重新运行太多了。

Answer 1

你可以这样做：

select
   sum(case when txnType in (3,20) then amount else 0 end) as pendingAmount,
   sum(case txnType when 11 then 1 else 0 end) as pendingReturn,
   sum(case txnType when 5 then 1 else 0 end) as pendingBlock
from
   Txn_Log
where
   gid = @gid

Answer 2

你能不能不做这样的事

SELECT sum(amount),count(1), txnType
FROM Txn_log
WHERE gid = @gid AND
    txnType in (3,5,11,20)
group by txnType

然后以编程的方式处理剩下的部分？