Dealing with new data for a type 2 slowly changing dimension using SQL

Here's a step-by-step guide for dealing with a Type 2 slowly changing dimension in SQL:

Step 1: Design your dimension table

Create a dimension table with columns such as primary key (ID), attributes, effective start date, effective end date, and a flag indicating the current record.

Step 2: Insert new records

When new data arrives, identify if it represents a new dimension member or an update to an existing member.
Use a combination of primary key matching and attribute comparison to determine this.
Insert the new data as a new record in the dimension table.
Set the effective start date to the current date and the effective end date to a future date (e.g., '9999-12-31').
Set the flag indicating the current record to true.

Step 3: Expire existing records

For dimension members that are being updated, mark their current records as expired.
Update the effective end date of the current record to the day before the new record's effective start date.
Set the flag indicating the current record to false.

Step 4: Handle overlaps

If there are overlapping date ranges, adjust the effective end dates of the affected records to ensure there are no gaps or overlaps.
For example, update the effective end date of the previous record to the day before the new record's effective start date.

Here's the SQL script incorporating these steps:

-- Step 2: Insert new records
INSERT INTO dimension_table (ID, attribute1, attribute2, effective_start_date, effective_end_date, is_current)
SELECT new_data.ID, new_data.attribute1, new_data.attribute2, CURRENT_DATE, '9999-12-31', true
FROM new_data
LEFT JOIN dimension_table ON new_data.ID = dimension_table.ID
WHERE dimension_table.ID IS NULL;

-- Step 3: Expire existing records
UPDATE dimension_table
SET effective_end_date = CURRENT_DATE - 1, is_current = false
WHERE ID IN (
    SELECT ID
    FROM new_data
    INNER JOIN dimension_table ON new_data.ID = dimension_table.ID
    WHERE new_data.attribute1 <> dimension_table.attribute1 OR new_data.attribute2 <> dimension_table.attribute2
);

-- Step 4: Handle overlaps
UPDATE dimension_table
SET effective_end_date = new_record.effective_start_date - 1
FROM (
    SELECT t1.ID, t1.effective_start_date
    FROM dimension_table t1
    INNER JOIN dimension_table t2 ON t1.ID = t2.ID
    WHERE t1.effective_start_date <= t2.effective_start_date
        AND t1.effective_end_date > t2.effective_start_date
        AND t2.is_current = true
) AS new_record
WHERE dimension_table.ID = new_record.ID;

Remember to replace dimension_table with the actual name of your dimension table, and adjust the column names and conditions to match your schema.

Be sure to test the script thoroughly and adapt it as needed for your specific database system and requirements.