SQLite 3.39

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CREATE TABLE t( col_A INT, col_B INT);
✓
INSERT INTO t(col_A, col_B) VALUES
 (111, 777),
 (222, 333),
 (222, 111),
 (444, 333),
 (222, 555),
 (333, 666),
 (333, 777)
✓
WITH t1 AS  
(
 SELECT col
   FROM (SELECT col_A AS col  FROM t UNION ALL
         SELECT col_B         FROM t) AS tt
  GROUP BY col
 HAVING COUNT(*)=1 
)
SELECT t.* 
  FROM t1
  JOIN t
    ON col IN ( col_A , col_B )


col_A
col_B


444
333

222
555

333
666