对同一字段的多个值进行聚合查询

Question

我使用的是 SQL-Server 2012，我有一张信用卡交易表，如下所示：

CardScheme  RespCode
=========================
Visa        Declined
MasterCard  Approved
MasterCard  Approved
MasterCard  Declined
Visa        Declined
MasterCard  Approved
MasterCard  Declined
Visa        Approved
Visa        Approved
MasterCard  Approved

我需要编写一个查询以获得以下结果

CardScheme  Declined  Approved
==============================
Visa        2         2 
MasterCard  2         4

请帮忙

Answer 1

由于您使用的是 SQL Server，因此您可以使用PIVOT函数将行转换为列：

select cardScheme, Declined, Approved
from yt
pivot
(
  count(RespCode)
  for RespCode in (Declined, Approved)
) piv;

参见SQL Fiddle with Demo