Question

我有一个名为$ var的PHP变量。我想根据$ var对结果进行排序。这个有效：

function myFunc(id: string) {
  id = "(";
};
// Note that this function does nothing, but it's not the issue here

let id: string = ")";

if (id !== '(')
  throw "Expected '(' in prototype";
// id is narrowed to "("

myFunc(id);

// now id and ')' does not have same type, hence the error
if (id !== ')')
  throw "Expected ')' in prototype";

但是当$ var为3时，我需要使用$sql = " SELECT myText1, myText2, someNumber FROM myTable ORDER BY CASE WHEN $var = 1 THEN myText1 WHEN $var = 2 THEN myText2 WHEN $var = 3 THEN someNumber END";按someNumber排序。所以我尝试了这个：

DESC

这会引发语法错误。我怎样才能做到这一点？

Answer 1

将其拆分为三个case表达式：

ORDER BY (CASE WHEN $var = 1 THEN myText1 END) ASC,
         (CASE WHEN $var = 2 THEN myText2 END) ASC,
         (CASE WHEN $var = 3 THEN someNumber END) DESC

ASC当然是可选的 - 只需与DESC签订合同。

更重要的是，这会删除自动转换。 CASE表达式只返回一种类型。混合使用不同类型时，会发生某种转换。该转换可能会影响值的排序。这解决了这个问题。

Answer 2

使用数学：

ORDER BY
CASE
    WHEN $var = 1 THEN myText1
    WHEN $var = 2 THEN myText2
    WHEN $var = 3 THEN 1000000 - someNumber
END

Answer 3

CASE只能返回一个表达式，因此您将遇到几种情况：

CASE WHEN $var = 1 THEN myText1 END,
CASE WHEN $var = 2 THEN myText2 END,
CASE WHEN $var = 3 THEN someNumber END DESC

OR，优化

CASE 
     WHEN $var = 1 THEN myText1,
     WHEN $var = 2 THEN myText2
END,
CASE WHEN $var = 3 THEN someNumber END DESC

MySQL - ORDER BY，CASE和DESC

3 个答案: