Mysql update column with value from another table

sql

conditional-statements

performance-optimization

data-safety

byAlex Kataev·Sep 10, 2024

If you need to update a column from one table with data from another, MySQL comes with a potent blend of INNER JOIN and UPDATE. Here's the magical incantation:

UPDATE target_table
INNER JOIN source_table ON target_table.id = source_table.foreign_id
SET target_table.column_to_update = source_table.column_to_copy;

This query will update target_table based on matches with source_table. Ensure target_table, source_table, id, foreign_id, column_to_update, and column_to_copy match your database schema.

Conditional updating: if... then... else...

For cases when you want to update data based on certain conditions, you can employ the power of IF within the SET clause:

UPDATE tableB AS t1
INNER JOIN tableA AS t2 ON t1.name = t2.name
SET t1.value = IF(t2.value > 0, t2.value, t1.value)
WHERE t1.name = 'Joe'; -- because 'Joe' deserves special attention!

With this, you're saying "If tableA.value is greater than 0, go ahead and update tableB.value with it. Else, let tableB.value rest in peace."

Optimizing performance for larger datasets

When dealing with gargantuan datasets, things can slow down. To prevent this tragic scenario, apply further constraints:

UPDATE tableB AS t1
INNER JOIN tableA AS t2 ON t1.name = t2.name
SET t1.value = t2.value
WHERE t2.value > 0 AND t1.id < 1000; -- because only 1000 top records deserve an update!

Here, we're only working with records in tableB where id is less than 1000 to keep things snappy.

Guard duty: Safeguarding your data

Safety first: When updating tables, think of yourself as the gatekeeper. Use conditions to limit the scope of your updates. This requires mastering the mighty WHERE clause and ensuring your join conditions are spot on. Remember, with great power (SQL) comes great responsibility.

Pitfall patrol: Common mistakes to dodge

The mass update mishap: If you leave out a WHERE clause, the update goes wild, affecting all records. Chaos ensues.
The cross-join calamity: Improper join conditions can lead to unprecedented data mix-up. Beware!
The name game: Using equivocal column names as join keys can cause unwanted data swapping. Stay alert!

Master techniques: Subquery updates

Sometimes, a subquery can work wonders, especially when wrangling data from multiple tables:

UPDATE tableB AS t1
SET value = (SELECT t2.value FROM tableA AS t2 WHERE t1.name = t2.name AND t2.value > 0)
WHERE t1.name = 'Joe'; -- because 'Joe' is not just another name!