-- https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/Concatenation-Operator.html
CREATE TABLE tab1 (col1 VARCHAR2(6), col2 CHAR(6),
                   col3 VARCHAR2(6), col4 CHAR(6));
INSERT INTO tab1 (col1,  col2,     col3,     col4)
          VALUES ('abc', 'def   ', 'ghi   ', 'jkl');
SELECT col1 || col2 || col3 || col4 "Concatenation"
  FROM tab1;