如何在 SQL 中创建布尔计算字段并加入该计算字段？答案

【问题标题】：How do I make a boolean calculated field in TSQL and join on that calculated field?如何在 SQL 中创建布尔计算字段并加入该计算字段？
【发布时间】：2009-01-06 17:12:48
【问题描述】：

首先查看此代码。我似乎应该对我有用，但事实并非如此！（惊喜！）

无论如何，这是我首先尝试的：

SELECT
Status as status,
Address as ip,
PCName as pc_name,
(Numbers.Phone = 'CPU/' + PCName) as cpu_contact,
(Numbers.Phone = 'PC/' + PCName) as pc_contact,
(Numbers.Phone = 'LOGIN/' + PCName) as login_contact,
FROM IPAddress
WHERE $where  --Generated In code
JOIN Numbers
  ON ('CPU/' + PCName = Numbers.Phone) 
  OR ('PC/' + PCName = Numbers.Phone) 
  OR ('LOGIN/' + PCName = Numbers.Phone)

所以我想要的是一些布尔计算字段并在类似条件下加入。我还希望结果折叠成单行。例如，我认为当前的设置会做这样的事情：

status ip  cpu_contact pc_contact login_contact
-----------------------------------------------
foo    bar true        false      false
foo    bar false       true       false
foo    bar false       false      true

显然我更愿意

status ip  cpu_contact pc_contact login_contact
-----------------------------------------------
foo    bar true        true       true

有什么想法吗？数据库重新设计不是一种选择。如果是的话，我会这样做:-)

【问题讨论】：

标签： sql sql-server tsql

【解决方案1】：

您可以使用GROUP BY 和SUM 折叠行：

SELECT
  Status as status, Address as ip, PCName as pc_name,
  cast(sum(case when (Numbers.Phone = 'CPU/' + PCName) then 1 else 0 end) as bit)
    as cpu_contact,
  cast(sum(case when (Numbers.Phone = 'PC/' + PCName) then 1 else 0 end)) as bit)
    as pc_contact,
  cast(sum(case when (Numbers.Phone = 'LOGIN/' + PCName) then 1 else 0 end) as bit)
    as login_contact,
FROM
  IPAddress
    JOIN Numbers ON 
      ('CPU/' + PCName = Numbers.Phone) OR ('PC/' + PCName = Numbers.Phone) OR 
      ('LOGIN/' + PCName = Numbers.Phone)
WHERE
  $where  --Generated In code
GROUP BY
  Status, Address, PCName

由于您在行之间进行逻辑或，因此零和为假，而任何大于 0 的值都为真。

【讨论】：

哇，这太复杂了！那好吧。我认为它会完成工作；它也很糟糕，因为我们有很多我没有列出的列，但我宁愿程序的一部分比所有部分都复杂。谢谢！

【解决方案2】：

您需要使用 Case/When 进行比较。在这种情况下，我将硬编码为 1 或 0，但 T-SQL 会将硬编码的数字转换为 int。如果你想要布尔（位），你需要手动转换，像这样......

Convert(Bit, Case When Numbers.Phone = 'CPU/' + PCName Then 1 Else 0 End) as cpu_contact,
Convert(Bit, Case When Numbers.Phone = 'PC/' + PCName Then 1 Else 0 End) as pc_contact,
Convert(Bit, Case When Numbers.Phone = 'LOGIN/' + PCName Then 1 Else 0 End) as login_contact,

【讨论】：

【解决方案3】：

SELECT
Status as status,
Address as ip,
PCName as pc_name,
case when sum(case when Numbers.Phone = 'CPU/' + PCName then 1 end) > 0 then 'true' else 'false' end as cpu_contact,
case when sum(case when Numbers.Phone = 'PC/' + PCName then 1 end) > 0 then 'true' else 'false' end as pc_contact,
case when sum(case when Numbers.Phone = 'LOGIN/' + PCName then 1 end) > 0 then 'true' else 'false' end as login_contact
FROM IPAddress
JOIN Numbers
  ON ('CPU/' + PCName = Numbers.Phone) 
  OR ('PC/' + PCName = Numbers.Phone) 
  OR ('LOGIN/' + PCName = Numbers.Phone)
WHERE -- your condition goes here
group by status, address, pc_name

【讨论】：