How Can I Efficiently Update a MySQL Table with Data from Another Table Using JOINs?

Efficiently Updating MySQL Tables with Values from Another Table

When faced with the need to update one MySQL table with information from another based solely on shared values, a common approach is to use a JOIN. However, achieving optimal performance with such queries requires careful consideration.

Suppose you have two tables with the following structures:

Original:

id	value
1	hello
2	fortune
3	my
4	old
5	friend

tobeupdated:

uniqueid	id	value
1	NULL	something
2	NULL	anything
3	NULL	old
4	NULL	friend
5	NULL	fortune

Your goal is to update the id column in tobeupdated based on the corresponding value in original. While the following query may seem straightforward:

UPDATE tobeupdated, original
SET tobeupdated.id = original.id
WHERE tobeupdated.value = original.value

it can be extremely slow, especially with large tables. This is because MySQL must perform a full table scan on both tables to find the matching rows.

Performance Optimization

To improve performance, a more efficient approach is to use a JOIN with an ON clause:

UPDATE tobeupdated
INNER JOIN original ON (tobeupdated.value = original.value)
SET tobeupdated.id = original.id

By using an INNER JOIN, MySQL only considers matching rows from both tables. Additionally, ensuring that indexes exist on the value columns in both tables can significantly reduce the time required for the search.

Simplified Syntax

For queries with identical key names in both tables, a USING clause can be used for simplicity:

UPDATE tobeupdated
INNER JOIN original USING (value)
SET tobeupdated.id = original.id

Additional Considerations

• Table Size: The size of the tables involved can impact performance. Consider optimizing the database design or using indexing techniques to improve performance.
• Cardinality: The cardinality (number of matching rows) between the tables can also affect performance. If there are many matching rows, the query may take longer.

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