为什么在Oracle和Mysql中不支持在同一选择中使用列别名？

Question

create table t1 (c1 integer);

select c1*3 temp, case when (temp <>3) then 1 else 0 end from t1;

Query fails in both Oracle and MySQL But why they doesn't support this type of queries?

I already went through the answers related to column alias. Everywhere its explained what is supported and what is not and reasoning about why not in where clause. But question is about why its not supported in select clause.

Answer 1

An alias can be used in a query select list to give a column a different name. You can use the alias in GROUP BY , ORDER BY , or HAVING clauses to refer to the column:

SELECT SQRT(a*b) AS root FROM tbl_name
  GROUP BY root HAVING root > 0;

SELECT id, COUNT(*) AS cnt FROM tbl_name
  GROUP BY id HAVING cnt > 0;

SELECT id AS 'Customer identity' FROM tbl_name;

Standard SQL disallows references to column aliases in a WHERE clause. This restriction is imposed because when the WHERE clause is evaluated, the column value may not yet have been determined. For example, the following query is illegal:

SELECT id, COUNT(*) AS cnt FROM tbl_name
  WHERE cnt > 0 GROUP BY id;

The WHERE clause determines which rows should be included in the GROUP BY clause, but it refers to the alias of a column value that is not known until after the rows have been selected, and grouped by the GROUP BY .

In the select list of a query, a quoted column alias can be specified using identifier or string quoting characters:

SELECT 1 AS `one`, 2 AS 'two';

Elsewhere in the statement, quoted references to the alias must use identifier quoting or the reference is treated as a string literal. For example, this statement groups by the values in column id, referenced using the alias a :

SELECT id AS 'a', COUNT(*) AS cnt FROM tbl_name
  GROUP BY `a`;

But this statement groups by the literal string 'a' and will not work as expected:

SELECT id AS 'a', COUNT(*) AS cnt FROM tbl_name
  GROUP BY 'a';

Source: https://docs.oracle.com/cd/E17952_01/refman-5.0-en/problems-with-alias.html

Answer 2

You can't use aliases in any sections except ORDER BY (in Oracle)

So you can do either:

select c1*3 temp, case when (c1*3 <>3) then 1 else 0 end from t1;

Or:

select temp, case when (temp <>3) then 1 else 0 end 
from (
select c1*3 temp from t1);