Question

I have a relational table with two foreign-keys pointing to same table.

The first foreign-key column should always be the minimum of a foreign property and the second one the maximum.

I am new to SQL Server and I read that a insert using the inserted temporary table should be used but how should I set the minimum and maximum relational childs not using a loop to iterate over the inserted entries.

Tables:

table Child
{
    int id;

    int value;
}

table Parent    
{
    int id;

    // foreign-keys A enforce minimum child value!
    int childA;
    int childB;
}

MySql Trigger

CREATE TRIGGER parent_beforeInsert
BEFORE INSERT ON Parent
    FOR each ROW
    BEGIN
        DECLARE childAValue AS INT;
        DECLARE childBValue AS INT;

        SET childAValue = (SELECT value FROM Child WHERE ID = NEW.childA);
        SET childBValue = (SELECT value FROM Child WHERE ID = NEW.childB);

        -- only check if b > a since reverse is wished behavior
        IF childBValue > childAValue
        BEGIN
            -- swap values              
            DECLARE newChildA AS INT = NEW.childA
            SET NEW.childA = NEW.childB;
            SET NEW.childB = newChildA;
        END;
    END;

Answer 1

Solved it using conditional select query.
Not so elegant, especially if many properties need to set but it works.

INSERT INTO Parent (childA_ID, childB_ID) 
    SELECT CASE WHEN childA.value <= childB.value THEN childA.ID ELSE childB.ID END AS childA_ID,
            CASE WHEN childA.value > childB.value THEN childA.ID ELSE childB.ID END AS childB_ID
        FROM inserted AS p, 
            Child AS childA, 
            Child AS childB,
        WHERE childA.ID = p.childA 
            AND childB.ID = p.childB;