sql - 为什么以下查询有不同的输出？

Question

尽管 2*3-5 = 1 为什么这两个查询有不同的输出？

SELECT * FROM table ORDER BY 1 asc, column2 desc;

对比

SELECT * FROM table ORDER BY 2*3-5 asc, column2 desc;

Answer 1

按结果集中的第一列排序，然后按column2排序

SELECT * FROM table ORDER BY 1 asc, column2 desc;

按常量表达式和column2排序:

SELECT * FROM table ORDER BY 2*3-5 asc, column2 desc;
<=>
SELECT * FROM table ORDER BY column2 desc;

SqlFiddleDemo

ORDER BY { column-Name | ColumnPosition | Expression }

ColumnPosition

An integer that identifies the number of the column in the SelectItems in the underlying query of the SELECT statement. ColumnPosition must be greater than 0 and not greater than the number of columns in the result table. In other words, if you want to order by a column, that column must be specified in the SELECT list.

请记住，在某些 RDBMS 中不允许按常量排序，例如 SQL Server:

SELECT * 
FROM tab
ORDER BY 2*3-5 asc, column2 desc;
-- A constant expression was encountered in the ORDER BY list, position 1.