如何在 SQL 中添加一列并在单个查询中同时基于其他列设置其值？

Question

For example, I have a Student table with columns named: id , name , marks_obtained , total_marks , etc.

I want to add a column named percentage whose values will be equal to ( marks_obtained / total_marks )*100.

How can I do the whole thing in a single query?

Answer 1

In Oracle 11g (since this is tagged Oracle) you can use a virtual column:

SQL Fiddle

Oracle 11g R2 Schema Setup :

CREATE TABLE Student ( id, name, marks_obtained, total_marks ) AS
          SELECT 1, 'Alice',  90, 100 FROM DUAL
UNION ALL SELECT 2, 'Bob',    72, 100 FROM DUAL
UNION ALL SELECT 3, 'Claire', 93, 105 FROM DUAL;

ALTER TABLE Student ADD (
  percentage NUMBER GENERATED ALWAYS AS ( marks_obtained * 100 / total_marks ) VIRTUAL
);

Query 1 :

SELECT * FROM Student

Results :

| ID |   NAME | MARKS_OBTAINED | TOTAL_MARKS |        PERCENTAGE |
|----|--------|----------------|-------------|-------------------|
|  1 |  Alice |             90 |         100 |                90 |
|  2 |    Bob |             72 |         100 |                72 |
|  3 | Claire |             93 |         105 | 88.57142857142857 |

Answer 2

If you don't want to add a computed column to the table you can do the calculation in the SQL which fetches your data, eg:

SELECT id,
       name,
       marks_obtained,
       total_marks,
       (marks_obtained / total_marks) * 100 AS PERCENTAGE
  FROM STUDENT

SQLFiddle here

Best of luck.