MySQL触发器的IF条件(IF condition on MySQL trigger)

 我有两个表（表A，表B）具有相同的字段（user_id，domi，ipiresia）。 用户可以在表A中添加值。我希望只有当表B中没有包含user_id时，才会在表B中自动插入这些值。 
 
CREATE TRIGGER new_value_added 
AFTER INSERT ON table_a
FOR EACH ROW
  IF NEW.user_id != user_id
  INSERT INTO table_b (user_id, domi, ipiresia)
  VALUES (NEW.user_id, NEW.domi, NEW.ipiresia);
 
 我知道IF语句是错误的，因为触发器不能按预期工作。 有帮助吗？ 

I have two tables (Table A, Table B) with the same fields (user_id, domi, ipiresia). Users can add values in Table A. I want these values to be inserted automatically in Table B BUT only when the user_id is not included in Table B. on.
 
CREATE TRIGGER new_value_added 
AFTER INSERT ON table_a
FOR EACH ROW
  IF NEW.user_id != user_id
  INSERT INTO table_b (user_id, domi, ipiresia)
  VALUES (NEW.user_id, NEW.domi, NEW.ipiresia);
 
I know that the IF statement is wrong because the trigger doesn't work as expected. Any help?

原文：https://stackoverflow.com/questions/33007001