如何在MySQL中别名字段或列？

Question

我正在尝试做这样的事情。但我得到一个未知的列错误：

SELECT SUM(field1 + field2) AS col1, col1 + field3 AS col3 from core 

基本上，我想只使用别名，以便我不需要执行前面执行的操作。这可能在MySQL？

Answer 1

考虑使用子查询，如：

SELECT col1 
,  col1 + field3 AS col3 
FROM (
     SELECT field1 + field2 as col1 
     ,  field3 
     from core 
     ) as SubQueryAlias 

Answer 2

简短的回答是否定的：

mysql> select 1 as a, a + 1 as b; 
ERROR 1054 (42S22): Unknown column 'a' in 'field list' 

postgresql# select 1 as a, a + 1 as b; 
ERROR: column "a" does not exist 

这就是说，一些SQL实现允许/ having子句使用其中/组的别名，如：

postgresql# select 1 as a group by a; -- 1 row 

Answer 3

select @code:= SUM(field1 + field2), @code+1 from abc;

但请注意以下内容（来自MySQL 5.6 docs）：

As a general rule, other than in SET statements, you should never assign a value to a user variable and read the value within the same statement. For example, to increment a variable, this is okay:

SET @a = @a + 1; 

For other statements, such as SELECT, you might get the results you expect, but this is not guaranteed. In the following statement, you might think that MySQL will evaluate @a first and then do an assignment second:

SELECT @a, @a:=@a+1, ...; 

However, the order of evaluation for expressions involving user variables is undefined.

因此，使用风险自负。

Answer 4

根据the spec拉维巴力的回答是不能保证总是工作，为“评价的顺序涉及用户变量是不确定的表述”。

我发现this answer后，我试图使用一个变量，并得到奇怪的结果。

Answer 5

select @code:= SUM(field1 + field2), (@code*1) from abc; 

@代号* 1隐蔽到数字表达式，你可以在任何地方使用像

select @code:= SUM(field1 + field2), (@code*1)+field3 from abc; 

Answer 6

可以select别名：

SELECT SUM(field1 + field2) AS col1, (select col1) + field3 AS col3 from core 

这工作。