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?
As long as you have suitable indexes in place this should work alright:
UPDATE table_a SET column_a_1 = (SELECT table_b.column_b_1 FROM table_b WHERE table_b.user_name = table_a.user_name ) , 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.