问题描述
我有两个表，有一个名为 user_name 的列，分别是 table_a、table_b.
I have two tables, with a same column named user_name, saying table_a, table_b.
我想，从table_b、column_b_1、column_b2复制到table_b1、column_a_1、column_a_2，分别是user_name相同的地方，在SQL语句中怎么做?
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?
推荐答案
只要你有合适的索引，这应该没问题:
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
)
sqlite3 中的 UPDATE 不支持 FROM 子句，这使得它比在其他 RDBMS 中.
UPDATE in sqlite3 does not support a FROM clause, which makes this a little more work than in other RDBMS.
如果性能不令人满意，另一种选择可能是使用 select 为 table_a 构建新行并与 table_a 连接到临时表中.然后从table_a中删除数据并从临时数据中重新填充.
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.
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