SQL & PL/SQL

We have two queries Query1 and Query2.I understood query2 output.if i remove group by deptno for query2 it is showing error  not a single group function.I believe group by is mandatory for LISTAGG function.In query 1 we are not using group by clause in a with clause.How query1 is running without group by clause and how it will do concatenation without group by clause?

Query1:

WITH t AS

  (SELECT 1 c1,'x' c2 FROM dual

  UNION

  SELECT 2 c1,'y' c2 FROM dual

  UNION

  SELECT 3 c1,'z' c2 FROM dual

  ) ,

  a AS

  (SELECT LENGTH(LISTAGG(c2, '') WITHIN GROUP (

  ORDER BY c2)) ln,

    LISTAGG(c2, '') WITHIN GROUP (

  ORDER BY c2) cont

  FROM t

  )

SELECT * from a

---

Query2:

select deptno,LISTAGG(sal, ',') WITHIN GROUP (

  ORDER BY sal) sal from emp

group by deptno

DEPTNO , SAL