How Can I Efficiently Copy Data Between SQL Tables Based on Matching Usernames?

Efficiently Transferring Data Between SQL Tables Using Matching Usernames

Your database contains two tables, table_a and table_b, each with a user_name column. The objective is to copy data from column_b_1 and column_b_2 in table_b to column_a_1 and column_a_2 in table_a, respectively, matching rows based on identical user_name values.

Assuming appropriate indexes are in place for optimal performance, a single SQL statement can accomplish this:

<code class="language-sql">UPDATE table_a
SET
    column_a_1 = (SELECT b.column_b_1 FROM table_b b WHERE b.user_name = table_a.user_name),
    column_a_2 = (SELECT b.column_b_2 FROM table_b b WHERE b.user_name = table_a.user_name)
WHERE
    EXISTS (SELECT 1 FROM table_b b WHERE b.user_name = table_a.user_name);</code>

This query employs correlated subqueries to fetch the correct values from table_b for each row in table_a. The EXISTS subquery ensures updates only happen for usernames present in both tables.

For significantly large datasets where performance is critical, consider this alternative strategy:

1. Construct a temporary table using a JOIN to combine relevant rows from table_a and table_b.
2. Remove existing data from table_a.
3. Insert the data from the temporary table into table_a.

Although this method involves more steps, it often delivers superior performance with extensive datasets.

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