Oracle's left join and where clauses errors CREATE TABLE "ATABLE1"
  (
    "COLUMN1" VARCHAR2(20 BYTE),
    "COLUMN2" VARCHAR2(20 BYTE)
  );

CREATE TABLE "ATABLE2"
  (
    "COLUMN1" VARCHAR2(20 BYTE),
    "COLUMN2" VARCHAR2(20 BYTE)
  );

Insert into ATABLE1 (COLUMN1,COLUMN2) values ('A','1');
Insert into ATABLE1 (COLUMN1,COLUMN2) values ('B','2');

Insert into ATABLE2 (COLUMN1,COLUMN2) values ('A',null);
Insert into ATABLE2 (COLUMN1,COLUMN2) values ('A','1');
Insert into ATABLE2 (COLUMN1,COLUMN2) values ('A','2');

select ATABLE1.column1, count(ATABLE2.column1) 
    from ATABLE1 Left OUTER JOIN ATABLE2 on ATABLE1.column1 = atable2.column1
    GROUP BY ATABLE1.column1;

Result

COLUMN1              COUNT(ATABLE2.COLUMN1) 
-------------------- ---------------------- 
A                    3                      
B                    0    


This works as expected. The thing is that I always want all rows from ATABLE1 to be shown and also apply some restrictions.

select ATABLE1.column1, count(ATABLE2.column1) 
    from ATABLE1 Left OUTER JOIN ATABLE2 on ATABLE1.column1 = atable2.column1
    where atable2.column2 = '1'
    GROUP BY ATABLE1.column1;


COLUMN1              COUNT(ATABLE2.COLUMN1) 
-------------------- ---------------------- 
A                    1                      


Why are not all columns from ATABLE1 being shown even with left join? How can I make them appear?

Thanks a lot in advance.
When you add WHERE filters to the optional/outer table, then you change the query into an INNER JOIN. You need to add the condition into the join, or derived table, or CTE.

select ATABLE1.column1, count(ATABLE2.column1) 
    from ATABLE1 Left OUTER JOIN ATABLE2
         on ATABLE1.column1 = atable2.column1 AND atable2.column2 = '1'
    GROUP BY ATABLE1.column1;