Mysql 如何在同一SQL语句中使用两个条件对字段求和两次

我有以下SQL语句：

Select SUM (amount) from accounts Where acctype = 0;

及

---

我想在一条SQL语句中找出两种情况下的金额之差。

这称为条件聚合：

SELECT
  SUM(case when acctype = 0 then amount else 0 end) AS sum0,
  SUM(case when acctype = 1 then amount else 0 end) AS sum1,
  SUM(case when acctype = 0 then amount else 0 end) -
  SUM(case when acctype = 1 then amount else 0 end) AS diff
FROM accounts;

select sum(case when acctype = 0 then amount
                when acctype = 1 then - amount
           end) as diff
from accounts;

---

这称为条件聚合：

SELECT
  SUM(case when acctype = 0 then amount else 0 end) AS sum0,
  SUM(case when acctype = 1 then amount else 0 end) AS sum1,
  SUM(case when acctype = 0 then amount else 0 end) -
  SUM(case when acctype = 1 then amount else 0 end) AS diff
FROM accounts;

select sum(case when acctype = 0 then amount
                when acctype = 1 then - amount
           end) as diff
from accounts;

---

您可以使用条件聚合：

SELECT
  SUM(case when acctype = 0 then amount else 0 end) AS sum0,
  SUM(case when acctype = 1 then amount else 0 end) AS sum1,
  SUM(case when acctype = 0 then amount else 0 end) -
  SUM(case when acctype = 1 then amount else 0 end) AS diff
FROM accounts;

select sum(case when acctype = 0 then amount
                when acctype = 1 then - amount
           end) as diff
from accounts;

---

您可以使用条件聚合：

SELECT
  SUM(case when acctype = 0 then amount else 0 end) AS sum0,
  SUM(case when acctype = 1 then amount else 0 end) AS sum1,
  SUM(case when acctype = 0 then amount else 0 end) -
  SUM(case when acctype = 1 then amount else 0 end) AS diff
FROM accounts;

select sum(case when acctype = 0 then amount
                when acctype = 1 then - amount
           end) as diff
from accounts;

---

如果您喜欢“hacks”：

从acctype输入（0,1）的帐户中选择SUM（金额*符号（acctype-0.5））如果您喜欢“hacks”：
从acctype输入（0,1）的帐户中选择SUM（金额*符号（acctype-0.5））我们是否可以使用-WHERE acctype之类的值介于0和1之间；。询问我的知识。@Belle：是的，你可以使用一个附加的
WHERE
子句，它甚至可以更快地得到结果。我更愿意使用<代码>，其中AccType在（0, 1）中，不过，我认为这更清楚（我们只对值0和1感兴趣，两者之间没有任何值）。谢谢。我们可以使用-WHERE acctype之类的值，介于0和1之间；。询问我的知识。@Belle：是的，你可以使用一个附加的
WHERE
子句，它甚至可以更快地得到结果。我更愿意使用<代码>，其中AccType在（0, 1）中，不过，我认为这更清楚（我们只对值0和1感兴趣，两者之间没有任何值）。谢谢你，尽管是我说的，但我认为你在这方面缺少了一个结尾case@GordonLinoff你的回答是正确的，正如RiggsFolly所说的，我们必须在diff之前结束，无论如何，非常感谢你，请把“结束”放在后面制定代码correct@Hamada：此解决方案可能比计算两个不同的
sum（）
s并将其相减更有效。这与我无关，但我认为您在这方面缺少了一个
end
case@Gordon林诺夫：你的回答是正确的，正如里格斯福利所说的，我们必须在不同的情况下先结束，无论如何，非常感谢你，请把“结束”的代码correct@Hamada：此解决方案可能比计算两个不同的
sum（）
s并将其相减更有效。