Sql 如果我在ORDERBY子句中添加一些列，这意味着什么？

我要问的是这个问题-

select * from emp order by sal + 2000;

select * from emp order by sal;

给出与此查询完全相同的结果-

select * from emp order by sal + 2000;

select * from emp order by sal;

那么在计算方面有什么不同呢？ sal+2000的订单是什么意思？ 这将如何影响结果

order by sal + 2000

上面的行可能没有用处，因为它可以执行以下两个操作之一：

它可能会在所有工资中增加2000英镑，然后结果也会相同，所有数据的顺序也会增加

“Order By”子句中可能不允许“Additon”操作

---

ORDERBY子句用于根据指定的列对结果进行排序 在您的情况下，默认情况下，您的结果将按Sal排序为ASC（升序）顺序

---

按order by执行计算对结果本身没有影响。

考虑查询中的以下结果集，按薪资升序排序（默认情况下）：

select name, salary from emp order by sal;

+------+--------+
| name | salary |
+------+--------+
| John | 10000  |
| Mike | 15000  |
| Joe  | 30000  |
+------+--------+

以下是使用sal+2000的顺序的结果集：

select name, salary, (salary+2000) as new_salary from emp order by new_salary;

+------+--------+------------+
| name | salary | new_salary |
+------+--------+------------+
| John | 10000  | 12000      |
| Mike | 15000  | 17000      |
| Joe  | 30000  | 32000      |
+------+--------+------------+

---

向薪资中添加2000不会改变您获得的顺序。

如果SAL存储为字符串，则这些查询不会给出相同的结果。

create table emp as
select  '90000' sal from dual union all
select '100000' sal from dual union all
select '110000' sal from dual;

select * from emp order by sal;

SAL
---
100000
110000
90000

select * from emp order by sal + 2000;

SAL
---
90000
100000
110000

create table emp2(id number, sal number not null);
select 1 id,  90000 sal from dual union all
select 2 id, 100000 sal from dual union all
select 3 id, 110000 sal from dual;
create index emp2_idx on emp2(sal);

explain plan for select * from emp2 order by sal;
select * from table(dbms_xplan.display(format => 'basic'));

Plan hash value: 1831800775

------------------------------------------------
| Id  | Operation                   | Name     |
------------------------------------------------
|   0 | SELECT STATEMENT            |          |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP2     |
|   2 |   INDEX FULL SCAN           | EMP2_IDX |
------------------------------------------------

explain plan for select * from emp2 order by sal + 2000;
select * from table(dbms_xplan.display(format => 'basic'));

Plan hash value: 2441141433

-----------------------------------
| Id  | Operation          | Name |
-----------------------------------
|   0 | SELECT STATEMENT   |      |
|   1 |  SORT ORDER BY     |      |
|   2 |   TABLE ACCESS FULL| EMP2 |
-----------------------------------

我希望这是不寻常的，但严格类型的数据模型太常见了

---

使用表达式可能会更改执行计划并避免索引。

create table emp as
select  '90000' sal from dual union all
select '100000' sal from dual union all
select '110000' sal from dual;

select * from emp order by sal;

SAL
---
100000
110000
90000

select * from emp order by sal + 2000;

SAL
---
90000
100000
110000

create table emp2(id number, sal number not null);
select 1 id,  90000 sal from dual union all
select 2 id, 100000 sal from dual union all
select 3 id, 110000 sal from dual;
create index emp2_idx on emp2(sal);

explain plan for select * from emp2 order by sal;
select * from table(dbms_xplan.display(format => 'basic'));

Plan hash value: 1831800775

------------------------------------------------
| Id  | Operation                   | Name     |
------------------------------------------------
|   0 | SELECT STATEMENT            |          |
|   1 |  TABLE ACCESS BY INDEX ROWID| EMP2     |
|   2 |   INDEX FULL SCAN           | EMP2_IDX |
------------------------------------------------

explain plan for select * from emp2 order by sal + 2000;
select * from table(dbms_xplan.display(format => 'basic'));

Plan hash value: 2441141433

-----------------------------------
| Id  | Operation          | Name |
-----------------------------------
|   0 | SELECT STATEMENT   |      |
|   1 |  SORT ORDER BY     |      |
|   2 |   TABLE ACCESS FULL| EMP2 |
-----------------------------------

---

通常还有其他更好的方法来避免索引，但有些人使用这种方法。

它毫无意义。您只是为所有

sal

@JohnWoo添加了一个

2000

的值，而不是为所有sal添加2000。它完全符合萨尔的命令。（结果透视）@szakwani我不知道你这是什么意思。据我所知，它将为工资添加一个值，但它不会显示

sal

列的新值，因为计算是在

ORDER BY

子句上进行的，而不是在

SELECT

子句上进行的。我的意思是，结果不会显示添加值。你在第一次评论中没有提到这一点。