Procedure integration

Question

Hello,
Am new to PL/SQL . Can someone please give me a piece of explanation, in what manners procedures, functions and packages can be used in queries?
1) Can function only appear in select row of query? What about procedures and packages?
Select func(var1,var2)
from dual
2) In which other situations can they be integrated ?
many thanks,

Answer 1

Procedures cannot be used in SQL because they do not return values.
Function can be used in any DML (SELECT, INSERT, DELETE, UPDATE) anywhere legal SQL expression can be used. However not all functions is allowed in SQL. E.g. functions that perform DML other than SELECT will be rejected. Consider following:
-- DROP TABLE T;
CREATE TABLE T(C NUMBER);
INSERT INTO T VALUES (0);
CREATE OR REPLACE PACKAGE P AS
     PROCEDURE PROC_NOT_OK;
     FUNCTION FUNC_NOT_OK RETURN NUMBER;
     FUNCTION FUNC_OK RETURN NUMBER;
END P;
CREATE OR REPLACE PACKAGE BODY P AS
     PROCEDURE PROC_NOT_OK IS
          BEGIN
               NULL;
          END;
     FUNCTION FUNC_NOT_OK RETURN NUMBER IS
          BEGIN
               DELETE FROM T;
               RETURN SQL%ROWCOUNT;
          END;
     FUNCTION FUNC_OK RETURN NUMBER IS
               nVAL NUMBER;
          BEGIN
               SELECT 1 INTO nVAL FROM DUAL;
               RETURN nVAL;
          END;
END P;
SYSTEM@TEST9> SELECT P.PROC_NOT_OK FROM DUAL;
SELECT P.PROC_NOT_OK FROM DUAL
ERROR at line 1:
ORA-00904: "P"."PROC_NOT_OK": invalid identifier
SYSTEM@TEST9> SELECT P.FUNC_NOT_OK FROM DUAL;
ERROR:
ORA-14551: cannot perform a DML operation inside a query
ORA-06512: at "SYSTEM.P", line 8
SELECT P.FUNC_NOT_OK FROM DUAL;
SYSTEM@TEST9> SELECT P.FUNC_OK FROM DUAL WHERE P.FUNC_OK <> 0 GROUP BY P.FUNC_OK;
FUNC_OK
1