将数据从一个表插入另一个表

Question

There is a one table,

City[city_id,state_id]

There was another table in same database,

Registration[reg_id,city_id]

Now, i have added one more column into 'Registration' table, so it is become as shown below,

    Registration[reg_id,city_id,state_id]

But, the problem is that values of the state_id column is '0'. So, How can i insert value of state_id column of "Registration" table from the "City" table according to matching city_id value of "Registration" table.

Answer 1

This will work in MySQL. In the background it instruct the server to make inner join on two tables.

UPDATE Registration r, City c SET r.state_id = c.state_id WHERE r.city_id = c.city_id;

Answer 2

As long as this query will be performed just once - you could use this not efficient, but readable query:

UPDATE Registration r
   SET state_id = (SELECT state_id
                     FROM City c
                    WHERE c.city_id = r.city_id)

Answer 3

不确定MySQL，但是可以在Oracle和SQL Server中完成：

UPDATE Registration r SET state_id = (SELECT state_id FROM City WHERE city_id = r.city_id)