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，是否有特殊的规则（使用它之前定义别名等）？

Answer 1

不，你应该使用它之前定义的别名，让你无论是：

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