Question

Certainly a noobish question, but I got to ask: 🙂 Assuming a column of type varchar and length 255 and the longest string stored in a row at this column shold have length 200. What happens, if I altered the columns length to less then 200? Would the strings all get “cut”?

Answer 1

By default, it will allow you to alter the column, it will truncate strings longer than the new length, and it will generate a warning.

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