SYSDATE

 In Oracle, SYSDATE is a built-in function that returns the current date and time from the system clock of the database server. It's one of the most commonly used functions to retrieve the current timestamp.

Here are the key details about SYSDATE:

1. Basic Syntax

SELECT SYSDATE FROM dual;

• dual is a special table in Oracle, used for queries that don’t require data from a table (i.e., retrieving system values like SYSDATE).

2. Return Value

• The SYSDATE function returns the current date and time in the format DD-MON-YY HH24:MI:SS, which includes both the date and the time.
• The return type of SYSDATE is DATE in Oracle, which contains both date and time information, even though the display format might not show the time if it's not explicitly specified.

3. Time Zone

• The SYSDATE function returns the date and time of the database server’s system clock. The time is provided in the time zone of the database server’s operating system, not necessarily the time zone of the user running the query.

4. Comparing with CURRENT_DATE and SYSDATE

• Both SYSDATE and CURRENT_DATE return the current date and time, but there’s a subtle difference:
• SYSDATE: Always returns the system’s current date and time.
• CURRENT_DATE: Returns the current date and time according to the session’s time zone.

5. Example Use Cases

• Current Date and Time: To get the current date and time from the system:

·        SELECT SYSDATE FROM dual;

• Extract Date or Time Components: You can use SYSDATE in combination with date functions to extract parts of the current timestamp. For example:
• To get the current year:

o   SELECT EXTRACT(YEAR FROM SYSDATE) FROM dual;

• To get the current month:

o   SELECT EXTRACT(MONTH FROM SYSDATE) FROM dual;

• Add/Subtract Time: You can perform arithmetic on SYSDATE to add or subtract time. For example:
• To get the date 5 days from today:

o   SELECT SYSDATE + 5 FROM dual;

• To subtract 3 hours from the current time:

o   SELECT SYSDATE - INTERVAL '3' HOUR FROM dual;

6. Performance Considerations

• SYSDATE is a non-deterministic function, meaning it can return different results for different invocations, even if the query runs at the same time.
• If you’re performing multiple queries in a single session, remember that the value of SYSDATE can change between queries if the system clock ticks forward.

7. Difference from SYSTIMESTAMP

• While SYSDATE returns a DATE data type (which includes both date and time), SYSTIMESTAMP returns a TIMESTAMP WITH TIME ZONE data type, which includes the date, time, and time zone information.

·        SELECT SYSTIMESTAMP FROM dual;

8. Practical Example:

Here’s an example of using SYSDATE in a query:

SELECT employee_id, hire_date, SYSDATE - hire_date AS days_since_hire

FROM employees;

In this query:

• SYSDATE - hire_date calculates the number of days an employee has been hired, based on the current system date.

Summary:

• SYSDATE is useful for getting the current date and time from the server's system clock.
• It's often used in queries to perform date arithmetic, comparisons, and in expressions involving current time.
• For timestamp and timezone awareness, SYSTIMESTAMP might be more appropriate, but for most applications, SYSDATE suffices.