How to Compare Data in SQL and Insert Rows if They Don't Match

Описание: Learn how to use SQL to compare rows based on a column and automatically insert new rows when values differ for better data management.
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This video is based on the question https://stackoverflow.com/q/64775215/ asked by the user 'Vikas J' ( https://stackoverflow.com/u/8534089/ ) and on the answer https://stackoverflow.com/a/64776362/ provided by the user 'SteveC' ( https://stackoverflow.com/u/6748758/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Compare Data in SQL and Insert Rows if They Don't Match

When working with SQL databases, you may encounter scenarios where you need to compare each row of data against the previous one based on a specific column. In this guide, we will walk through a problem regarding SQL Server 2017 where you need to compare the POS_ID values row by row, and if a difference is detected, insert new rows in the table with predefined statuses.

The Problem

You have a table that contains the following columns: POS_ID, Term Code, Status, and IsActive. You need to iterate over the table and compare each POS_ID with the one in the previous row. If the POS_ID changes, two new rows must be inserted:

The first new row should have the Status as "In Progress" and IsActive as true, maintaining the Term Code from the new POS_ID.

The second new row should have the Status as "discontinue" and IsActive as false, also using the Term Code from the new POS_ID.

Initial Data Setup

Let’s first create the table and insert some initial data:

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The Solution

To achieve the required functionality of inserting new rows based on the conditions above, we can use Common Table Expressions (CTEs) in SQL along with the LAG() function. This function allows us to access data from the previous row and compare it without additional iteration.

Step 1: Create a Temporary Table

First, we create a temporary table to simulate our data for easier testing:

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Step 2: Insert New Rows Condition

Next, we will write an insert statement to add the new rows based on the comparison of POS_ID values:

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Step 3: Query the Updated Data

After executing the above insert statement, we can now query the updated data to see the results:

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Example Output

What you can expect to see after running the code are several new rows inserted based on the POS_ID changes. Here’s an example output:

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Conclusion

Using SQL’s LAG() function with CTEs offers an efficient way to handle complex data comparisons and allows you to manipulate your database entries dynamically. By following the steps outlined in this guide, you can effectively insert new rows into your SQL table based on the comparison of values in a specified column.

Feel free to explore and modify this approach to fit your particular use case!