Apply "ORDER BY" On A "UNION" (Mysql)

Answer :

SELECT * FROM ( (SELECT * FROM user_relation WHERE from_user_id = 1) UNION (SELECT * FROM user_relation WHERE to_user_id = 1) ) AS i ORDER BY trust_degree 

You have to assign an alias to your select. But in this case a UNION is not necessary and could be replaced by a simple OR, as @Karoly Horvath points out in his comment. The resulting query would look like this:

SELECT   *  FROM user_relation  WHERE from_user_id = 1 OR to_user_id = 1  ORDER BY trust_degree 

It is written in the documentation of UNION:

To apply ORDER BY or LIMIT to an individual SELECT, place the clause inside the parentheses that enclose the SELECT:

(SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10) UNION (SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10); 

...

Use of ORDER BY for individual SELECT statements implies nothing about the order in which the rows appear in the final result because UNION by default produces an unordered set of rows.

...

To use an ORDER BY or LIMIT clause to sort or limit the entire UNION result, parenthesize the individual SELECT statements and place the ORDER BY or LIMIT after the last one. The following example uses both clauses:

(SELECT a FROM t1 WHERE a=10 AND B=1) UNION (SELECT a FROM t2 WHERE a=11 AND B=2) ORDER BY a LIMIT 10; 

A statement without parentheses is equivalent to one parenthesized as just shown.

By applying the above information to your query it becomes:

(SELECT * FROM user_relation WHERE from_user_id = 1) UNION (SELECT * FROM user_relation WHERE to_user_id = 1) ORDER BY trust_degree