sql: how to update table values from another table with the same user name?

Question

I have two tables, with a same column named user_name, saying table_a, table_b. I want to, copy from table_b, column_b_1, column_b2, to table_b1, column_a_1, column_a_2, respectively, where the user_name is the same, how to do it in sql statement?

Answer 1

As long as you have suitable indexes in place this should work alright:

UPDATE table_a
SET
      table_a.column_a_1 = (SELECT table_b.column_b_1 
                            FROM table_b
                            WHERE table_b.user_name = table_a.user_name )
    , table_a.column_a_2 = (SELECT table_b.column_b_2
                            FROM table_b
                            WHERE table_b.user_name = table_a.user_name )
WHERE
    EXISTS (
        SELECT *
        FROM table_b
        WHERE table_b.user_name = table_a.user_name
    )

UPDATE in sqlite3 does not support a FROM clause, which makes this a little more work than in other RDBMS.

If performance is not satisfactory, another option might be to build up new rows for table_a using a select and join with table_a into a temporary table. Then delete the data from table_a and repopulate from the temporary.