How can I use safe SQL practices when manipulating data?

You wake up in a cold sweat, body shaking, and filled with a weight that can be described as a herd of African elephants. You muster up the strength to drive away the herd and stagger over to your laptop with eyes like a newborn puppy. With arms like a harp string plucked, you timidly log into your church’s production instance of Rock. Everything is still there. It was all a dream.

For some of us, this may not have been a dream. We have actually ruined our production instances of Rock by modifying data with SQL. Thankfully, this isn’t always irreversible. With proper backups and some tears in our eyes, we are typically able to undo the atrocity that’s been unleashed on our Rock environment. But it’s a painful experience, and in hindsight, it’s always one that could have been avoided. This Rock Solid Data post explores three best practices for safely using SQL’s Data Manipulation Language (DML)—UPDATE, DELETE, and INSERT—so you can modify data with confidence.

Before executing an UPDATE, DELETE, and INSERT, start with a SELECT statement to preview the data you plan to modify. Make sure that you get the SELECT statement working perfectly and then convert the SELECT statement to your DML function of choice. If you only want to update 500 rows, make sure your SELECT statement only 500 rows first. This easy habit can help prevent any unintended data modifications.

Another safeguard is using transactions. Transactions let you test changes to your database before they become permanent. It works by first executing a BEGIN TRANSACTION function, which opens a temporary session. Next, you execute your SQL modifications within the new session, but they are not made permanent immediately. This gives you an opportunity to verify the results using a SELECT statement. Finally, you can either use COMMIT or ROLLBACK to either finalize or cancel the pending changes to your data. This two-step process creates a safety net that allows you to avoid mistakes that can arise from writing new SQL. You can find more information about the syntax on this page and the other pages in its subfolder.

The last way to safeguard your data is the easiest: consider whether you need to use an UPDATE, DELETE, and INSERT. Sometimes, Rock’s built in tools can accomplish the same goal without directly changing the database. So, before you go to use UPDATE, DELETE, and INSERT, be sure to think critically about your problem and consider if there is an alternative way to solve it.

One of the most powerful and complex SQL statements is MERGE, which combines the UPDATE, DELETE, and INSERT functions into one operation. The MERGE statement is highly efficient for matching data between tables but requires extreme caution to use. It works by matching records between a source and target table and modifying data based on the shared or unique values. While it can be a powerful tool to modify your data, it comes with great responsibility due to the amount of SQL required and the possible complexity of the matching condition. It would be best to stick with your current methods of modifying data if they function well already. You can learn more about MERGE on this page (scroll down to 'Examples' to see it in action).

Using the DML functions responsibly can mean the difference between a seamless update and a disastrous data loss. Writing a SELECT statement first, using transactions to review changes, and considering alternative solutions can eliminate costly mistakes. By using these strategies, you can work confidently with your data and avoid the nightmare of waking up to a destroyed database.

If you're feeling down about SQL after this, let one of Chipʼs hits bring you back up!