将数据插入到数据来自两个 SELECT 查询的表中

Question

I have a table that I need to insert data to. The table has 2 columns:

ClassStudents (table)

• ClassId
• StudentId

The data I want to insert can be found from 2 tables:

Classes (table)

Students (table)

I need to get relevant Classes.Id and relevant Students.Id and insert them to ClassStudents table. I can get the relevant data from each table but I don't know how to insert those data to ClassStudents. The data of the 2 tables won't have the same number of records: Classes will have fewer records than Students, but I need to insert a combination of all Classes.Id to Students.Id and insert to ClassStudents table all those combinations.

I am thinking I need a CROSS JOIN and then use that to insert but I don't know how to or if its even allowed to CROSS JOIN two select queries (one from Classes to get all relevant data and one from Students for also, relevant data).

Answer 1

You don't provide any requisit sample data and the result expected.

However you probably (and if not you'll need to clarify) just need a union

Insert into ClassStudents (ClassId, StudentId)
select ClassId, StudentId
from Classes
union
select ClassId, StudentId
from Students

If you need to keep duplicates, use union all .