Oracle 11g Release 2

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CREATE TABLE table1(ID INT,column_name1 VARCHAR2(3))
CREATE TABLE table2(ID INT,column_name1 VARCHAR2(3))
CREATE TABLE table3(ID INT,column_name2 VARCHAR2(3))
INSERT INTO table1
 SELECT 26, NULL FROM dual UNION ALL
 SELECT 30, NULL FROM dual UNION ALL
 SELECT 34, NULL FROM dual  
3 rows affected

INSERT INTO table2
 SELECT 26, 'fix' FROM dual UNION ALL
 SELECT 30, 'var' FROM dual UNION ALL
 SELECT 34, 'fix' FROM dual  
3 rows affected

INSERT INTO table3
 SELECT 26, 'fix' FROM dual UNION ALL
 SELECT 30,  NULL FROM dual UNION ALL
 SELECT 34, 'fix' FROM dual 
3 rows affected

-- The SELECT statement(ID column added) within the question returns duplicated ID values
SELECT tt.* 
  FROM table1 t1
  JOIN ( SELECT * FROM table2  
         UNION  
         SELECT * FROM table3 ) tt 
    ON tt.ID = t1.ID


ID
COLUMN_NAME1


26
fix

30
var

30
null

34
fix


-- The SELECT statement(ID column added) within the answer does not return duplicated ID values
SELECT t3.ID, NVL(t3.column_name2, t2.column_name1)
  FROM table2 t2
  JOIN table3 t3
    ON t3.ID = t2.ID


ID
NVL(T3.COLUMN_NAME2,T2.COLUMN_NAME1)


26
fix

30
var

34
fix


UPDATE table1 t1
   SET t1.column_name1 = (SELECT NVL(t3.column_name2, t2.column_name1)
                            FROM table2 t2
                            JOIN table3 t3
                              ON t3.ID = t2.ID
                           WHERE t3.ID = t1.ID)  
 WHERE EXISTS
           ( SELECT *
               FROM table2 t2
               JOIN table3 t3
                 ON t3.ID = t2.ID
              WHERE t3.ID = t1.ID )
3 rows affected

SELECT * FROM table1


ID
COLUMN_NAME1


26
fix

30
var

34
fix