UPDATE table1 t1
   SET t1.text1 = (SELECT text2
                     FROM table2
                    WHERE x1 = t1.x1);

table1
x  text1
 1   '1_111'
 2   '1_222'

table2
x  text2
 1   '2_1111'
 1   '2_1112'
 2   '2_2222'

UPDATE table1 t1
   SET t1.text1 = (SELECT text2
                     FROM table2
                    WHERE x1 = t1.x1
                      AND text2 IS NOT NULL);

CREATE TABLE table1
(
    x     NUMBER,
    text1 VARCHAR2( 10 )
);

CREATE TABLE table2
(
    x     NUMBER,
    text2 VARCHAR2( 10 )
);

INSERT INTO table1 (x, text1) VALUES ( 131333 , 'xxxx');
INSERT INTO table1 (x, text1) VALUES ( 373627 , 'xqqq');
INSERT INTO table1 (x, text1) VALUES ( 131313 , NULL);

INSERT INTO table2 (x, text2) VALUES ( 131333 , '23fmkd');
INSERT INTO table2 (x, text2) VALUES ( 373627 , 'iutnccn');

COMMIT;

SELECT * FROM table1;

-- Результат
--         X TEXT1
------------ ----------
--    131333 xxxx
--    373627 xqqq
--    131313

SELECT * FROM table2;

-- Результат
--         X TEXT2
------------ ----------
--    131333 23fmkd
--    373627 iutnccn

UPDATE table1 t1
   SET t1.text1 = (SELECT text2
                     FROM table2
                    WHERE x = t1.x
                      AND text2 IS NOT NULL);

SELECT * FROM table1;

-- Результат
--         X TEXT1
------------ ----------
--    131333 23fmkd
--    373627 iutnccn
--    131313

UPDATE table1
   SET t1.text1 = (SELECT text2
                     FROM table2
                    WHERE x = table1.x
                      AND text2 IS NOT NULL);