我们可以从 MySQL 中的另一个表向一个表添加一列吗？

Mysql> create table FirstTable
   -> ( 
   -> UserId int,
   -> UserName varchar(20)
   -> );
Query OK, 0 rows affected (1.48 sec)

mysql> create table SecondTable
   -> (
   -> UserId int,
   -> UserAge int
   -> );
Query OK, 0 rows affected (1.57 sec)

mysql> ALTER TABLE FirstTable ADD COLUMN Age TINYINT UNSIGNED DEFAULT 0;
Query OK, 0 rows affected (1.53 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> UPDATE FirstTable tbl1
   -> INNER JOIN SecondTable tbl2 ON tbl1.UserId = tbl2.UserId
   -> SET tbl1.Age = tbl2.UserAge;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0

mysql> desc FirstTable;

+----------+---------------------+------+-----+---------+-------+
| Field    | Type                | Null | Key | Default | Extra |
+----------+---------------------+------+-----+---------+-------+
| UserId   | int(11)             | YES  |     | NULL    |       |
| UserName | varchar(20)         | YES  |     | NULL    |       |
| Age      | tinyint(3) unsigned | YES  |     | 0       |       |
+----------+---------------------+------+-----+---------+-------+
3 rows in set (0.53 sec)