problem using ORDER BY in postgres with UNION clause

Question

I am trying to run this query in postgresql

SELECT DISTINCT users."uid", tax."tid"
FROM tax
LEFT JOIN users ON tax."uid" = users."uid"

UNION 

SELECT DISTINCT users."uid", tax."tid"
FROM tax 
LEFT JOIN users ON tax."uid" = users."uid"
ORDER BY users."uid", tax."tid"

but I come across this error

missing FROM-clause entry for table "users"
LINE 10: ORDER BY users."uid", tax."tid"

however, when I run these queries I come across no problem and the results are as expected :-

SELECT DISTINCT users."uid", tax."tid"
FROM tax 
LEFT JOIN users ON tax."uid" = users."uid"
ORDER BY users."uid", tax."tid"

SELECT DISTINCT users."uid", tax."tid"
FROM tax
LEFT JOIN users ON tax."uid" = users."uid"
ORDER BY users."uid", tax."tid"

SELECT DISTINCT users."uid", tax."tid"
FROM tax
LEFT JOIN users ON tax."uid" = users."uid"

UNION 

SELECT DISTINCT users."uid", tax."tid"
FROM tax 
LEFT JOIN users ON tax."uid" = users."uid"

what am I doing wrong here?

Answer 1

Leave out the table aliases and refer to the columns only by their table aliases:

SELECT users."uid", tax."tid"
FROM tax LEFT JOIN
     users
     ON tax."uid" = users."uid"
UNION 
SELECT users."uid", tax."tid"
FROM tax LEFT JOIN
     users
     ON tax."uid" = users."uid"
ORDER BY "uid", "tid";

The SELECT DISTINCT is redundant, because UNION removes duplicates.