Question

当然,这是一个不容置疑的问题,但我要问: 假设每栏252个字体和总长度为255个栏目,以及该栏内一个行长储存的最长体为200个长度。如果我将一栏长度改为少于200,会发生什么? 所有这一切是否“执行”?

Answer 1

否则,它将允许你改变这个栏目,将限制比新长度更长的阵列,并产生警告。

mysql> create table t (v varchar(20));
Query OK, 0 rows affected (0.06 sec)

mysql> insert into t values ( 12345678901234567890 );
Query OK, 1 row affected (0.00 sec)

mysql> alter table t modify column v varchar(10);
Query OK, 1 row affected, 1 warning (0.04 sec)
Records: 1  Duplicates: 0  Warnings: 1

mysql> show warnings;
+---------+------+----------------------------------------+
| Level   | Code | Message                                |
+---------+------+----------------------------------------+
| Warning | 1265 | Data truncated for column  v  at row 1 | 
+---------+------+----------------------------------------+
1 row in set (0.00 sec)

mysql> select * from t;
+------------+
| v          |
+------------+
| 1234567890 | 
+------------+
1 row in set (0.00 sec)

If you have the SQL mode STRICT_ALL_TABLES or STRICT_TRANS_TABLES set, the warning becomes an error and the ALTER will fail.

mysql> alter table t modify column v varchar(10);
ERROR 1265 (01000): Data truncated for column  v  at row 1

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