如何从一个表的字段中获取其主键在另一个表中的2个可能字段中

Question

I have 2 sql tables

    CREATE TABLE users(userID INTEGER PRIMARY KEY AUTOINCREMENT, alias TEXT NOT NULL)
    CREATE TABLE friends(userID1 INTEGER, userID2 INTEGER, PRIMARY KEY(userID1, userID2))

When 2 users want to be friends, I create a new relationship btw the 2 by inserting a row in the friends table. The order for the userID of the 2 friends in the row do not matter but it should be unique, whatever the order.

Question 1: My second table do not currently reflect what I want. I mean I can do this and it's fine while I don't want it to be:

    INSERT INTO friends VALUES(1,2);
    INSERT INTO friends VALUES(2,1);

Is it possible to enforce this constraint in my sql table, or should I check with a query if there is not already a relationship in the reversed order.

Question 2: What is the most optimal sql query to get the list of friends ( userID and alias ) if I have a userID , if I have for example:

    INSERT INTO users VALUES(1, 'A');
    INSERT INTO users VALUES(2, 'B');
    INSERT INTO users VALUES(3, 'C');
    INSERT INTO users VALUES(4, 'D');
    INSERT INTO friends VALUES(1, 2);
    INSERT INTO friends VALUES(2, 3);
    INSERT INTO friends VALUES(4, 1);
    INSERT INTO friends VALUES(2, 4);

I want:

For userID=1: (2, B) and (4, D)
For userID=2: (1, A), (3, C) and (4, D)
For userID=3: (2, B)
For userID=4: (1, A) and (2, B)

Bonus question: Can I improve the tables to better reflect what I want?

Thanks a lot!

Answer 1

One question at a time. Change your insert from this structure:

insert into table
(field1, field2, etc)
values
(value1, value2, etc)

to this structure

insert into table
(field1, field2, etc)
select distinct value1, value2, etc
from someSmallTable
where not exists
(subquery to check for existing rows)