用別名計算

Question

簡單問題：
我可以使用別名來計算嗎？是

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

合法，是否有特殊的規則（使用它之前定義別名等）？

Answer 1

不，你應該使用它之前定義的別名，讓你無論是：

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