问题
I have 3 tables. I want the school table to set the data of major_subject and get the minor_subject on the result.
And also set automatically units 3.0 for major_subject since this table has no column.
I tried to use UNION but I get error since major_subject column is not the same with minor_subject table.
school major_subject
| school_id | school_name | | subj_name | date_offered |
----------------------------- ---------------------------------
| 1 | schoolA | | Business101 | 2021/01/01 |
| 2 | schoolB | | Marketing101 | 2021/01/01 |
| 3 | schoolC |
minor_subject
| school_id | subj_name | units | date_offered |
----------------------------------------------------
| 1 | Math | 1.0 | 2021/01/01 |
| 1 | English | 1.0 | 2021/01/01 |
| 1 | Science | 1.0 | 2021/01/01 |
| 2 | History | 2.0 | 2021/01/01 |
And the result table would be like:
| school_id | subj_name | units | date_offered |
---------------------------------------------------------
| 1 | Business101 | 3.0 | 2021/01/01 |
| 1 | Marketing101 | 3.0 | 2021/01/01 |
| 1 | Math | 1.0 | 2021/01/01 |
| 1 | English | 1.0 | 2021/01/01 |
| 1 | Science | 1.0 | 2021/01/01 |
| 2 | Business101 | 3.0 | 2021/01/01 |
| 2 | Marketing101 | 3.0 | 2021/01/01 |
| 2 | History | 2.0 | 2021/01/01 |
| 3 | Business101 | 3.0 | 2021/01/01 |
| 3 | Marketing101 | 3.0 | 2021/01/01 |
回答1:
You can CROSS join school to major_subject to get all the combinations of the rows of the 2 tables and then with UNION ALL add the rows from minor_subject:
SELECT s.school_id, m.subj_name, 3.0 units, m.date_offered
FROM school s CROSS JOIN major_subject m
UNION ALL
SELECT school_id, subj_name, units, date_offered
FROM minor_subject
ORDER BY school_id
See the demo.
Results:
> school_id | subj_name | units | date_offered
> --------: | :----------- | ----: | :-----------
> 1 | Business101 | 3.0 | 2021-01-01
> 1 | Marketing101 | 3.0 | 2021-01-01
> 1 | Math | 1.0 | 2021-01-01
> 1 | English | 1.0 | 2021-01-01
> 1 | Science | 1.0 | 2021-01-01
> 2 | History | 2.0 | 2021-01-01
> 2 | Marketing101 | 3.0 | 2021-01-01
> 2 | Business101 | 3.0 | 2021-01-01
> 3 | Business101 | 3.0 | 2021-01-01
> 3 | Marketing101 | 3.0 | 2021-01-01
来源：https://stackoverflow.com/questions/65248274/how-to-combine-multiple-tables-with-different-columns