用别名计算

Question

简单问题：
我可以使用别名来计算吗？是

SELECT example1 / example2 AS ratio
     , SQRT(ratio) AS squareroot
FROM example_table

合法且有特殊规则（在使用之前定义别名等）吗？

Answer 1

不，您应该在使用前定义别名，所以要么：

select example1 / example2 as ratio
     , sqrt(example1 / example2) as squareroot 
from example_table

或

select ratio
     , sqrt(ratio) as squareroot
from ( 
  select example1 / example2 as ratio 
  from example_table
) as tbl