sql检查对象是否正在使用触发器

I have a table lets say tableA already filled with var_code and start_time and end_time and I want to insert a value into another table but before I insert it I want to check if this var_code is already in use in a specific time defined in tableA. To find the relation between the new object and time I had to join three tables. When I try to insert something I get an error message: ERROR: query has no destination for result data What do I do wrong?I have a table lets say tableA already filled w