===== Valid Time Temporal =====
==== Create Tables ====
Create a period using existing columns
<code>CREATE TABLE student_modules (
  id         NUMBER(10) NOT NULL,
  student_id NUMBER(10) NOT NULL,
  module_id  NUMBER(10) NOT NULL,
  start_date DATE,
  end_date   DATE,
  PERIOD FOR student_module_period (start_date, end_date)
);</code>
Create a period with system generated hidden columns
<code>CREATE TABLE student_modules (
  id         NUMBER(10) NOT NULL,
  student_id NUMBER(10) NOT NULL,
  module_id  NUMBER(10) NOT NULL,
  start_date DATE,
  end_date   DATE,
  PERIOD FOR student_module_period
  );</code>

==== Viewing Data ====
We can now create **AS OF PERIOD FOR** queries.\\
The below query finds all students on a currently active course.
<code>SELECT * FROM student_modules AS OF PERIOD FOR student_module_period SYSDATE sm;</code>
Can also create **VERSIONS PERIOD FOR ... BETWEEN** queries.\\
The below query finds all students that were active in the last week.
<code>SELECT * FROM student_modules VERSIONS PERIOD FOR 
              student_module_period BETWEEN TRUNC(SYSDATE)-7 AND TRUNC(SYSDATE);</code>
              
==== Using Flashback ====
You can also use the DBMS_FLASHBACK_ARCHIVE.ENABLE_AT_VALID_TIME procedure
<code>BEGIN
  DBMS_FLASHBACK_ARCHIVE.ENABLE_AT_VALID_TIME ('ASOF','20-JUL-15 12.00.01 PM');
END;

BEGIN
  DBMS_FLASHBACK_ARCHIVE.ENABLE_AT_VALID_TIME ('CURRENT');
END;

BEGIN
  DBMS_FLASHBACK_ARCHIVE.ENABLE_AT_VALID_TIME ('ALL');
END;</code>