In Oracle SQL, there are several methods to calculate the first day of the month from a given date. Below are examples of these methods along with their expected outputs if we assume that today's date is January 4, 2024 (2024-01-04):
Example 1: Using TRUNC with Date
The TRUNC function can be used to truncate a date to the first day of the month.
SELECT TRUNC(SYSDATE, 'MM') AS first_day_of_month FROM dual;
Expected Output:
FIRST_DAY_OF_MONTH ------------------- 2024-01-01
Explanation: The TRUNC function removes the time component and sets the day to the first of the month for the given date.
Example 2: Arithmetic with LAST_DAY
Use the LAST_DAY function to get the last day of the previous month and then add one day.
SELECT LAST_DAY(ADD_MONTHS(SYSDATE, -1)) + 1 AS first_day_of_month FROM dual;
Expected Output:
FIRST_DAY_OF_MONTH ------------------- 2024-01-01
Explanation: ADD_MONTHS subtracts one month, LAST_DAY finds the last day of that month, and adding 1 gets us to the first day of the current month.
Example 3: Using TO_DATE and TO_CHAR
Reconstruct the date by extracting the year and month, then appending the first day.
SELECT TO_DATE(TO_CHAR(SYSDATE, 'YYYY-MM') || '-01', 'YYYY-MM-DD') AS first_day_of_month FROM dual;
Expected Output:
FIRST_DAY_OF_MONTH ------------------- 2024-01-01
Explanation: TO_CHAR formats the date as a string to include only the year and month. By adding the '-01', we specify the first day and then convert it back to a date.
Example 4: Using EXTRACT
Extract the year and month parts of the date and then construct the first day of the month.
SELECT TO_DATE(EXTRACT(YEAR FROM SYSDATE) || '-' || EXTRACT(MONTH FROM SYSDATE) || '-01', 'YYYY-MM-DD') AS first_day_of_month FROM dual;
Expected Output:
FIRST_DAY_OF_MONTH ------------------- 2024-01-01
Explanation: EXTRACT is used to get the numerical year and month values from SYSDATE. These are then concatenated with '-01' to form a string representing the first day of the month, which is converted back to a date.
These expected outputs assume that the NLS_DATE_FORMAT session parameter is set to display dates in the YYYY-MM-DD format. If the format is different, the output may appear in a different date format. You can set this format in your session using the ALTER SESSION SET NLS_DATE_FORMAT='YYYY-MM-DD'; command before executing the queries.
