Example Codes for Subtracting 1 Day from a Timestamp Date:

PostgreSQL: PostgreSQL is a specific type of relational database management system (RDBMS) that uses SQL. This explanation applies to other SQL-based databases as well, but the syntax might vary slightly.

Date: In SQL, "date" refers to a data type that stores calendar information like year, month, and day. "Timestamp" is another data type that includes both date and time information (hours, minutes, seconds).

Subtracting a Day: Here are two common methods to subtract one day from a timestamp date in SQL:

1. Direct Subtraction: This method works for dates and timestamps. You can directly subtract the number 1 from the date or timestamp value.

• Example: SELECT your_date_field - 1 AS yesterday FROM your_table;
  This query subtracts 1 from the your_date_field and aliases the result as "yesterday".

1. DATEADD Function (for some databases): Some databases, like Microsoft SQL Server, offer functions specifically for date manipulation. The DATEADD function allows you to add or subtract a specified number of days from a date/time value.

Example Codes for Subtracting 1 Day from a Timestamp Date:

This example uses the your_date_field as the timestamp and subtracts 1 day.

SELECT your_date_field - 1 AS yesterday
FROM your_table;

Example 2: Using DATEADD Function (For databases like SQL Server)

This example assumes you have a DATEADD function and uses it to subtract 1 day from your_date_field.

SELECT DATEADD(DAY, -1, your_date_field) AS yesterday
FROM your_table;

Explanation:

• Both examples use SELECT to retrieve data.
• your_date_field represents the actual column name holding your timestamp data. Replace it with the actual column name in your table.
• In example 1, we directly subtract 1 from your_date_field. This works for most databases that recognize timestamps as a data type.
• In example 2, we use the DATEADD(DAY, -1, your_date_field) function.
  • DATEADD is the function name (check your specific database documentation for function availability).
  • DAY specifies the unit of time we want to subtract (days in this case).
  • -1 indicates the number of days to subtract (negative for subtracting).
  • your_date_field is the timestamp value from which we want to subtract.
• Both examples use AS yesterday to alias the result as "yesterday" for easier reading.

This method involves temporarily converting the timestamp to a date type, subtracting the day, and then casting it back to a timestamp.

SELECT CAST(your_date_field AS DATE) - INTERVAL '1 DAY' AS yesterday
FROM your_table;

• CAST(your_date_field AS DATE) converts the timestamp to a date type, allowing direct date manipulation.
• INTERVAL '1 DAY' is a literal representing a one-day interval.
• The subtraction subtracts the one-day interval from the cast date.
• Finally, the result is cast back to a timestamp.

Using Date Functions and Subtraction:

Some databases offer specific date manipulation functions. Here's an example using DATE_SUB (check your database documentation for available functions):

SELECT DATE_SUB(your_date_field, INTERVAL 1 DAY) AS yesterday
FROM your_table;

• DATE_SUB is a function that subtracts a specified interval from a date/time value.
• your_date_field is the timestamp value.
• INTERVAL 1 DAY specifies the interval to subtract (one day).

Using a Subquery:

This method involves a subquery to achieve the subtraction. It might be less efficient but can be useful in specific scenarios.

SELECT your_date_field AS original_date, your_date_field - INTERVAL '1 DAY' AS yesterday
FROM your_table;

• This query retrieves both the original your_date_field and the result of subtracting one day using INTERVAL '1 DAY'.

Choosing the Right Method:

• The direct subtraction method (first example) is generally the simplest and most widely compatible approach.
• If your database offers specific date manipulation functions like DATEADD or DATE_SUB, they might be more efficient.
• The CAST and subtraction method offers flexibility but might be slightly less efficient than direct subtraction.
• The subquery method is less common but could be useful in specific situations where you need to see both the original and modified values.