What are pencil hardness grading scales?

2H -> H -> F -> HB -> B -> 2B

H means Hard (hard-tipped)
B means Bold (soft-tipped)
F means Fine

Info: from left to right, writing is thicker on the paper.

Generally, it would say that H (Hard) is designed for drafting and S (Soft) is designed for writing.

What is implicit commit?

After every DDL operation, implicit commit occurs such as while defining a new table.

e.g.
CREATE TABLE HR.EXAMPLE(ABC char(1));

Also we call it as "auto commit" operation.

blob vs bfile data types

BLOB (binary large objects) stores unstructured binary large objects. It is often used for graphic images, video clips, and sounds. The BLOB data type stores the content inside the Oracle database.

BFILE (binary file) is also used to large binary file stored outside the database. Oracle can read the file only, not modify it. Oracle requires appropriate operating system level read permissions on the file.

Ref: Oracle SQL Interactive Workbook (Alice Rischert)

How to do CTAS in Oracle?

CTAS stands for Create Table As Select

e.g.
SQL>
CREATE TABLE HR.EMPLOYEES_NEW
AS
   SELECT * FROM HR.EMPLOYEES;

-> By default, only "NOT_NULL" constraints copy to new table (HR.EMPLOYEES_NEW) if it has (HR.EMPLOYEES). (PK, FK, index, etc. does not copy)

CTAS is faster because UNDO and REDO data does not generate. (It use NOLOGGING AND PARALEL methods)

NULLS LAST keyword usage

-> HR.EMPLOYEES will be test table.
SQL> 
DESC HR.EMPLOYEES;

e.g.
To populate all data
SQL> 
SELECT * FROM HR.EMPLOYEES;

To manipulate data by "COMMISSION_PCT" column descending order.
SQL> 
  SELECT *
    FROM HR.EMPLOYEES
ORDER BY COMMISSION_PCT DESC;

-> This is wrong query cause "NULL" columns show on top. Therefore, "NULLS LAST" keyword should be used.
SQL>
  SELECT *
    FROM HR.EMPLOYEES
ORDER BY COMMISSION_PCT DESC NULLS LAST;

ORA-01495 solution

Action
SQL>
ANALYZE TABLE HR.EMPLOYEES LIST CHAINED ROWS INTO CHAINED_ROWS;

Error
ORA-01495: specified chain row table not found

Solution
SQL>
CREATE TABLE HR.CHAINED_ROWS
(
   OWNER_NAME          VARCHAR2 (30),
   TABLE_NAME          VARCHAR2 (30),
   CLUSTER_NAME        VARCHAR2 (30),
   PARTITION_NAME      VARCHAR2 (30),
   SUBPARTITION_NAME   VARCHAR2 (30),
   HEAD_ROWID          ROWID,
   ANALYZE_TIMESTAMP   DATE
);

ORA-02270 solution

Preparation
SQL>
create table A (id number, soid number);

SQL>
create table B (id number);

Action
SQL> 
alter table A
add constraint SOID_B_FK
foreign key (soid)
references B (id);

Error
ORA-02270: no matching unique or primary key for this column-list

Reason
Parent table (B) does not have any primary key for matching with child table (A)

Solution
SQL>
alter table B
add constraint ID_PK 
primary key (id);

ORA-00959 solution

Action
SQL> 
CREATE TABLE HR.EXAMPLE(ABC char(1));

Error
ORA-00959: tablespace '_$deleted$40$0' does not exist

Reason
Defined default tablespace is not available any more for HR user.

Solution
SQL>
ALTER USER HR DEFAULT TABLESPACE NEW_USERS;

ORA-02449 solution

Preparation
SQL>
-- A table is child table
create table A (id number, soid number);

SQL>
-- B table is parent table
create table B (id number); 

SQL>
alter table B
add constraint ID_B_PK 
primary key (id);

SQL>
alter table A
add constraint ID_A_PK 
primary key (id);

SQL>
alter table A
add constraint SOID_B_FK
foreign key (soid)
references B (id);

Action
SQL> 
drop table B;

Error
ORA-02449: unique/primary keys in table referenced by foreign keys

Solution
SQL>
drop table B cascade constraints;

Result
By "cascade constraints" keywords, only SOID_B_FK foreign key removed from Table A, ID_A_PK primary key remains for Table A.

ORA-01720 solution

Preparation
"employees" table exists in HR schema

"v_employees" view exists in HR2 schema;
SQL> grant select on hr.employees_new to hr2;
SQL> create view hr2.v_employees as select * from hr.employees;

User wants to see H2.V_EMPLOYEES view with HR3 user.
Action
SQL> grant select on hr2.v_employees to hr3;

Error
ORA-01720: grant option does not exist for 'HR.EMPLOYEES'

Solution
Append grant option to HR.EMPLOYEES table for HR2 user
SQL> grant select on hr.employees to hr2 with grant option;

Then, grant select permission to HR3 user
SQL> grant select on hr2.v_employees to hr3;

Result
SQL> select * from HR2.v_employees;

How to use SQL Tuning Advisor in Oracle SQL Developer?

Oracle SQL Developer -> Connections -> Right mouse click on desired database connection name (e.g. XE) -> Click "Open SQL Worksheet" -> Input and select SQL code block -> Right mouse click then select "SQL Tuning Advisor" or press "CTRL+F12" -> Lastly, analyze the result.

How to take screenshot on top window quickly?

e.g.
Alt key + Print Screen key -> Then CTRL + V in "mspaint.exe" -> Save it.

How to see both Oracle VM Virtual Box 32bit/64bit options?

e.g. If there are only 32 bit Linux distro in the OS list, you should enable virtualization support in BIOS

* BIOS -> CMOS Setup Utility -> Virtualization Technology -> [Enabled]

e.g. bulk insert

SET TIMING ON
/
CREATE TABLE HR.EMPLOYEES_TEST
AS
   SELECT *
     FROM HR.EMPLOYEES
    WHERE ROWNUM < 0;
/
DECLARE
   TYPE TABLE_TYP IS TABLE OF HR.EMPLOYEES%ROWTYPE;

   TABLE_ROWS   TABLE_TYP := TABLE_TYP ();

   CURSOR TEST_CURSOR
   IS
      SELECT * FROM HR.EMPLOYEES;

   V_COUNT      NUMBER;
BEGIN
   SELECT COUNT (1) INTO V_COUNT FROM HR.EMPLOYEES;

   DBMS_OUTPUT.PUT_LINE ('TOTAL NUMBER OF ROWS: ' || V_COUNT);

   OPEN TEST_CURSOR;

   LOOP
      FETCH TEST_CURSOR BULK COLLECT INTO TABLE_ROWS LIMIT 10;

      FORALL A IN TABLE_ROWS.FIRST .. TABLE_ROWS.LAST
         INSERT INTO HR.EMPLOYEES_TEST
              VALUES TABLE_ROWS (A);

      COMMIT;
      DBMS_OUTPUT.PUT_LINE (
         'Looping, TEST_CURSOR%rowcount = ' || TEST_CURSOR%ROWCOUNT);
      EXIT WHEN TEST_CURSOR%NOTFOUND;
   
   --EXIT WHEN TEST_CURSOR%ROWCOUNT > 1000000;

   END LOOP;

   DBMS_OUTPUT.PUT_LINE (
      'Exited, TEST_CURSOR%rowcount = ' || TEST_CURSOR%ROWCOUNT);

   CLOSE TEST_CURSOR;
END;
/

[ADVICE] Free Database Monitoring Tool

You may use following free tool instead of Oracle EM (Enterprise Manager)

• ASH Viewer (Active Session History)

Naming conventions for database objects

index -> [SHORT_TABLE_NAME]_[SHORT_COLUMN_NAME]_[INDEX_TYPE]
e.g. EMP_DEPARTMENT_IX

constraint -> [SHORT_TABLE_NAME]_[SHORT_COLUMN_NAME]_[CONSTRAINT_TYPE]
e.g. 
NOT NULL const. -> EMP_EMAIL_NN
FOREIGN KEY (Referential Integrity) const. 
Source table: EMPLOYEES
Column name of source table: DEPARTMENT_ID
-> EMP_DEPT_FK

trigger -> [TABLE_NAME]_TRG
e.g. HR.EMPLOYEES_TRG

sequence -> [TABLE_NAME]_SEQ
e.g. HR.EMPLOYEES_SEQ

type -> [TABLE_NAME]_TYP
e.g. HR.EMPLOYEES_TYP

How to debug code in Toad?

e.g.
Toad for Oracle -> Database -> Schema Browser -> Click HR -> Click Procedures -> Double click to "PROC_EXEC" procedure -> Set breakpoint or Add Watch (optional) -> Press Shift+F7 or Click Debug-Trace Into -> Confirm -> Click Execute

How to grap and drop table columns in Toad?

e.g.
Toad for Oracle -> Type HR.EMPLOYEES then Press F4 in Editor -> Select desired columns by CTRL key-> Drag and drop to Editor by mouse -> Columns should show in Editor

optimizer_index_cost_adj parameter

Purpose
The purpose of optimizer_index_cost_adj parameter is to force use index even the cost is high. (decides large-table full-table scans or index range scans)

Usage
Current setting:
SQL> show parameter optimizer_index_cost_adj;
-- default is 100
-- ranges between 1 and 10000

(options like -> 10 | 100 | 1000)
Change setting permanently:
add this into init.ora file;
*.optimizer_index_cost_adj=10
-- "10" is to force it uses index
-- "100" is the default value
-- "1000" is not to force and it tries to different method (large-table full-table) to search data  

Then restart the instance to perform.

Change setting for current instance - temporary:
SQL> alter system set optimizer_index_cost_adj = 10;

Result
SQL> show parameter optimizer_index_cost_adj;
-- output; 10

How to define primary key on existed table?

e.g.
steps;
lock table -> add new column for primary key -> update data with counter -> define sequence -> define trigger -> lastly, define primary key -> recompile invalid objects (optional)

SQL>
LOCK TABLE HR.EMPLOYEES_NEW IN EXCLUSIVE MODE NOWAIT
/
ALTER TABLE HR.EMPLOYEES_NEW
ADD (ID NUMBER)
/
DECLARE
   str_exec      VARCHAR2 (250);
   counter       NUMBER := 0;
   last_seq_id   NUMBER;
BEGIN
   FOR cur_node IN (  SELECT ROWID, id
                        FROM HR.EMPLOYEES_NEW
                    ORDER BY HIRE_DATE ASC)
   LOOP
      BEGIN
         str_exec :=
               'update HR.EMPLOYEES_NEW set id =  '
            || counter
            || ' where rowid = '
            || ''''
            || cur_node.ROWID
            || '''';

         --DBMS_OUTPUT.put_line (str_exec);

         EXECUTE IMMEDIATE str_exec;

         COMMIT;

         counter := counter + 1;
      END;
   END LOOP;

   SELECT ID 
     INTO last_seq_id
     FROM (  SELECT ID 
               FROM HR.EMPLOYEES_NEW
              WHERE ID IS NOT NULL
           ORDER BY ID DESC)
    WHERE ROWNUM < 2;

   str_exec :=
         'CREATE SEQUENCE HR.EMPLOYEES_NEW_ID_SEQ START WITH '
      || last_seq_id
      || ' MAXVALUE 999999999999999999999999999999 MINVALUE '
      || last_seq_id
      || ' NOCYCLE NOCACHE NOORDER';

   --DBMS_OUTPUT.put_line (str_exec);

   EXECUTE IMMEDIATE str_exec;
END;
/
CREATE OR REPLACE TRIGGER HR.TRG_EMPLOYEES_NEW_ID
   BEFORE INSERT
   ON HR.EMPLOYEES_NEW
   FOR EACH ROW
BEGIN
   IF :new.ID IS NULL
   THEN
      :new.ID := HR.EMPLOYEES_NEW_ID_SEQ.NEXTVAL;
   END IF;
END TRG_EMPLOYEES_NEW_ID;
/
ALTER TABLE HR.EMPLOYEES_NEW ADD
CONSTRAINT EMPLOYEES_NEW_PK
 PRIMARY KEY (ID)
 ENABLE
 VALIDATE
/
EXEC SYS.UTL_RECOMP.recomp_parallel(4)
/