creating a 'trend' data using SQL (Repeating a value day by day until it changes) – Sql

The Problem:

Given a table with historical price data for an item, consisting of a ‘price’ column and a ‘date’ column, the task is to generate a new table that repeats the price for each day, starting from the ‘date’ column, until the price changes or a specified end date is reached. The objective is to create a trend graph in Metabase, which requires consistently day-by-day data. This repeated data will be used to visualize trends and patterns in the price of the item over time.

The Solutions:

Solution 1: Using Calendar Table and Outer Apply

To create a "trend" data that repeats values day by day until it changes, we can utilize a calendar table and an outer apply. Here’s how:

-- Create a calendar table
WITH Dates AS (
    SELECT date
    FROM calendar
    WHERE date BETWEEN MIN(date) AND GETDATE()
)

-- Join the calendar table with the price history table
SELECT
    d.date,
    COALESCE(p.price, 0) AS price
FROM Dates d
LEFT JOIN PriceHistory p ON d.date = p.date;

1. Create a Calendar Table (Dates):

   • Use a calendar table or create one on the fly using a recursive CTE. This table contains all the dates you’re interested in.

2. Join Tables:

   • Join the calendar table (Dates) with the price history table (PriceHistory) using the date column.

3. Apply COALESCE Function:

   • Use the COALESCE function to populate the price column with the price from the PriceHistory table. If no price is found for a particular date, it returns 0 as the default value.

This query will return a table with dates as rows and the corresponding price for each date. The COALESCE function ensures that you have a price for every date, even if it remains unchanged from the previous day.

Solution 2: Generating Dates and Joining with Prices

To generate the desired trend data, we can modify the approach to create a recursive CTE for generating dates day-by-day and then join it with the original data to assign prices to those dates.

1. Generate Dates Day-by-Day:
   • Create a recursive CTE named dates that generates dates day-by-day.

WITH RECURSIVE dates AS (
    -- Start with the earliest date from your_table_name
    SELECT date FROM your_table_name
    ORDER BY date LIMIT 1

    UNION ALL

    -- Generate the next date
    SELECT DATE_ADD(date, INTERVAL 1 DAY) FROM dates
    WHERE DATE_ADD(date, INTERVAL 1 DAY) <= CURRENT_DATE
)

2. Join Dates with Prices:
   • Create a CTE named prices by joining the dates CTE with the original data (your_table_name) using a left join. This will assign prices to each generated date.

WITH prices AS (
    SELECT d.date, COALESCE(t2.price, 0) AS price
    FROM dates d
    LEFT JOIN your_table_name t2 ON d.date = t2.date
)

3. Select Price and Date:
   • Select the price and date columns from the prices CTE to retrieve the result with prices repeated day-by-day until they change.

SELECT price, date FROM prices;

By using this approach, the result will consist of rows containing prices repeated day-by-day for each date in the generated range, providing the trend data you need for visualization in Metabase or other reporting tools. This method offers a more versatile solution that can be easily adapted to different scenarios and data structures.

Video Explanation: