StartDate invalid identifier

I have two SELECT statment which I connect using UNION and I get error

ORA-00904: "PUSD2"."STARTDATE": invalid identifier

Since I create first one SELECT statment and run and I get result, and second also, so I run separatly and it works perfect. But when I use UNION I get error pusd2.StartDate invalid identifier.

SELECT * FROM project_users_schedule_dates pusd1
 WHERE pusd1.UserID = 196 AND 
  pusd1.StartDate >= '2020-04-27' AND pusd1.EndDate <= '2020-06-06'  
   UNION 
   SELECT * FROM project_users_schedule_dates pusd2
    WHERE pusd2.UserID = 196 AND 
    pusd2.StartDate <= '2020-04-27' AND pusd2. EndDate >= '2020-06-06' 
     ORDER  BY pusd2.StartDate

What is wrong here ? What I made wrong ?

Table structure project_users_schedule_dates

Answer

The error likely stems from using an alias in the second union in your ORDER BY clause. However, I might suggest writing your query this way instead:

SELECT DISTINCT *
FROM project_users_schedule_dates
WHERE
    UserID = 196 AND 
    (StartDate >= '2020-04-27' AND EndDate <= '2020-06-06' OR
     StartDate <= '2020-04-27' AND EndDate >= '2020-06-06');

This approach brings together the critera from both sides of the union in a single WHERE clause. It then filters duplicates using DISTINCT, instead of the union. Note that in general doing SELECT * in a union query is not desirable, because it makes it impossible to see which columns are being included.