Sql UPDATE SET one column to be equal to a value in a related table referenced by a different column?

NULL values: Handle with care!

Consider the presence of NULL in the update process. If your goal is to update only where ColumnToBeUpdated in TableA is NULL, make it explicit in the WHERE clause:

AND TableA.ColumnToBeUpdated IS NULL; -- like a friendly ghost, only visible if NULL
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By targeting rows where values are yet to be set, you ensure you're not overwriting any precious existing data.

The magic of INNER JOIN in updates

Sometimes, INNER JOIN can make updates efficient and less laggy. Here's how:

UPDATE TableA
SET TableA.ColumnToBeUpdated = RelatedTable.ColumnOfInterest
FROM TableA
INNER JOIN RelatedTable ON TableA.DifferentColumn = RelatedTable.RelatedColumn -- handshake between tables
WHERE TableA.ColumnToBeUpdated IS NULL; -- NULL here, move along
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The INNER JOIN syncs records in both tables, while the WHERE acts as a filter, only updating where necessary, avoiding the sin of data overwrite.

JOINs: High reward, high risk

JOINs essentially marries two tables. And just like marriage, a minor mistake can mess things up. Always double-check join conditions and run a SELECT query to know what you're updating:

SELECT TableA.*, RelatedTable.ColumnOfInterest
FROM TableA
INNER JOIN RelatedTable ON TableA.DifferentColumn = RelatedTable.RelatedColumn -- all aboard the join train!
WHERE TableA.ColumnToBeUpdated IS NULL; -- NULL spots? Yes, please!
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Inspect this output carefully to avoid waking up in a data nightmare.

Database-specific syntax: Required course

Different SQL dialects have their unique syntax updates. Keep the language documentation at your disposal, or run a test in a sandbox environment before you jump onto the production server.

Subqueries and performance: The balancing act

Keep in mind, subqueries are not always the fastest kids on the block. Face-off between performance(🐎) and correctness(🎯), often comes down to the nature of your dataset and indexing strategy. Choose your weapon wisely.

A word on data integrity

Just like in the Avengers, safeguarding the universe, updating data also requires a shield, called transactions. This ensures that those sneaky partial updates don’t cause havoc in your data world.

Test your query: Don't let the bug bite!

After your query is ready for action, give it a good test run. A sandbox environment is your best friend here. Don't let your production database be the guinea pig. Ensure that it runs as expected and when assured, let it loose in the wild.

Customized conditions: Making your query fit!

Requirements vary. Maybe you want to update rows that comply with a certain condition, or maybe not if another column is set to a specific value. Speak these rules out loud in your WHERE clause to make your UPDATE statement fit the needs of your data.