如果以前不存在要输入的值，如何创建SQL触发器以在另一个表中输入新值？

Question

I'm working with Oracle SQL, and I have two tables. A table of conversations and a table of messages, and a message can't be added unless a conversation exists first. So when a new message wants to be added to the message table, I need to have a trigger to check is a conversation for that message exists yet or not, and if it doens't it will add a new conversation values to the conversation table for the message. "Warning: Trigger created with compilation errors." But I don't seem to see my error. I was hoping to get any insight here on what I can do to make this code work better.

CREATE OR REPLACE TRIGGER CreateConversation
    BEFORE INSERT ON Messages
    FOR EACH ROW
    BEGIN
        dbms_output.put_line('made it inside');
        IF NOT EXISTS (select * FROM Conversation WHERE convID =     new.convID AND msgID = new.msgID ) 
            THEN
            dbms_output.put_line('Conversation does not exist!');
            INSERT INTO Conversation VALUES(:new.convID, :new.msgID);
        END IF;
    END;
/

Answer 1

You can not use EXISTS in PL/SQL; if you need to check the existence of related records, try with something like this (not tested):

CREATE OR REPLACE TRIGGER CreateConversation 
    BEFORE INSERT ON Messages
    FOR EACH ROW
declare    
    vCount number;
    BEGIN
        dbms_output.put_line('made it inside');
        select count(1) 
        into vCount
        FROM Conversation
        WHERE convID = :new.convID
          AND msgID = :new.msgID;
        IF vCount = 0
            THEN
            dbms_output.put_line('Conversation does not exist!');
            INSERT INTO Conversation VALUES(:new.convID, :new.msgID);
        END IF;
    END;
/