sql - Insert into with union -

i have problem. there 3 tables: t1, t2, t_target. t1 , t2 table have many different columns need id column both. t_target table has id column of course , another: project_no.

there ids appears in t1 , t2 too, don't want create duplicates between them, if id appears in both table have inserted t_target once if in t_target it's allowed act twice. other of criteria every newly inserted id must value 21 in 'project_no' column. so, e.g.:

t1:

id 2548 2566 2569 2843 2888 ... 

t2:

id 2557 2566 2569 2700 2913 2994 3018 5426 ... 

t_target:

id     project_no 2976   1 3331   7 4049   7 5426   8 5915   3 6253   10 ... 

and result want see:

t_target:

id     project_no 2548   21 2557   21 2566   21 2569   21 2700   21 2843   21 2888   21 2913   21 2976   1 2994   21 2018   21 3331   7 4049   7 5426   8 5426   21 5915   3 6253   10 ... 

so, tried code (it important here "not null" criteria because both of t_target columns primary key):

insert t_target (id, project_no)   select (select id   t1 id not null  union   select id   t2 id not null), 21  select * t_target 

the error message: "msg 512, level 16, state 1, line 2 subquery returned more 1 value. not permitted when subquery follows =, !=, <, <= , >, >= or when subquery used expression. statement has been terminated."

then tried values statement instead of first select , parentheses error same.

there similar problem: mysql query: how insert union? solution doesn't work me because indicates syntax error between value , select.

please, give me hand. thank you!

this should need

insert t_target             (id,              project_no) select id,        21   t1  id not null union select id,        21   t2  id not null