Oracle 21c

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DECLARE
   a INT DEFAULT 1;
   b INT DEFAULT 2;
BEGIN
  SELECT x, y
  INTO a,b 
  FROM (SELECT 100 AS x, 200 AS y FROM dual) s; 
  
   DBMS_OUTPUT.PUT_LINE(a || '    ' || b);
END;
/
1 rows affected

dbms_output:
100    200

DECLARE
   a INT DEFAULT 1;
   b INT DEFAULT 2;
BEGIN
  SELECT x, y
  INTO a,b 
  FROM (SELECT 100 AS x, 200 AS y FROM dual
        UNION ALL 
        SELECT 100 AS x, 200 AS y FROM dual
       ) s; 
  
   DBMS_OUTPUT.PUT_LINE(a || '    ' || b);
END;
/
ORA-01422: exact fetch returns more than requested number of rows
ORA-06512: at line 5

DECLARE
   a INT DEFAULT 1;
   b INT DEFAULT 2;
BEGIN
  SELECT x, y
  INTO a,b 
  FROM (SELECT 100 AS x, 200 AS y FROM dual) s
  WHERE 1=2; 
  
   DBMS_OUTPUT.PUT_LINE(a || '    ' || b);
END;
/
ORA-01403: no data found
ORA-06512: at line 5