子查询是重用变量的唯一选择吗？
我想以这种形式使用 MySQL：
SELECT 1 AS one, one*2 AS two
因为它比前者更短、更甜蜜，
SELECT one*2 AS two FROM ( SELECT 1 AS one ) AS sub1
但前者似乎不起作用，因为它期望一个是一列。
有没有更简单的方法可以在不使用子查询的情况下实现这种效果？
不，SELECT 2 AS Two 不是一个选项。 ;)
I'd like to use MySQL in this form:
SELECT 1 AS one, one*2 AS two
because it's shorter and sweeter than
SELECT one*2 AS two FROM ( SELECT 1 AS one ) AS sub1
but the former doesn't seem to work because it expects one to be a column.
Is there any easier way to accomplish this effect without subqueries?
And no, SELECT 2 AS two is not an option. ;)
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从 SQL 语言的角度考虑此 SQL 代码
（为什么不呢；mysql 在遵守 ISO/ANSI SQL 标准方面有着良好的记录），您的
one不是一个变量；而是一个变量。相反，它是一个列关联名称。您不能在具有相同范围的 SELECT 子句中使用相关名称，因此会出现错误。FWIW，您的“更短更甜”语法在使用 MS Access 数据库引擎时实际上确实有效 - 也许您是从那里学到的？遗憾的是，Access 数据库引擎在遵守标准方面的记录不佳。据说要花很长时间才能忘记Access-speak并学习SQL代码；）
Considering this SQL code
from the perspective of SQL the language (and why not; mysql has a good track record of compliance with the ISO/ANSI SQL Standards), your
oneis not a variable; rather it is a column correlation name. You cannot use the correlation name in theSELECTclause with the same scope, hence the error.FWIW your 'shorter and sweeter' syntax does actually work when using the MS Access Database Engine -- is that where you learned it, perchance? Sadly, the Access Database Engine has a poor track record of compliance with the Standards. It is said to take a long time to un-learn Access-speak and learn SQL code ;)
用户定义变量
user-defined variables