Here we give int char no width, and the system assigns it a width by default.
m indicates the maximum display width. The maximum effective display width is 255. The display width is independent of the range of values that the storage size or type contains
Let's do the test.
Copy Code code as follows:
MySQL (root@localhost:test 03:19:00) >create Table C (
-> ID int NOT NULL,
-> name char not NULL);
Query OK, 0 rows affected (0.25 sec)
MySQL (root@localhost:test 03:19:34) >desc C;
+-------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| ID | Int (11) | NO | | NULL | |
| name | char (1) | NO | | NULL | |
+-------+---------+------+-----+---------+-------+
2 rows in Set (0.00 sec)
So we can see here, the system will automatically give our data type a default broadband value, where this width is in fact only in the role of Zerofill can play a certain role. Below we'll see what the other defaults are,
Copy Code code as follows:
MySQL (root@localhost:test 03:34:53) >alter table C Modify ID smallint;
Query OK, 0 rows affected (0.05 sec)
records:0 duplicates:0 warnings:0
MySQL (root@localhost:test 03:39:39) >desc C;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID | smallint (6) | YES | | NULL | |
| name | varchar (10) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in Set (0.00 sec)
MySQL (root@localhost:test 03:39:44) >alter table C Modify ID bigint;
Query OK, 4 rows affected (0.23 sec)
Records:4 duplicates:0 warnings:0
MySQL (root@localhost:test 03:40:12) >desc C;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID | bigint (20) | YES | | NULL | |
| name | varchar (10) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in Set (0.01 sec)
Here we look at the case where the insertion value is greater than the range of values of the data type:
Copy Code code as follows:
MySQL (root@localhost:test 03:25:58) >insert into C values (+, ' Chen ');
Query OK, 1 row affected, 2 warnings (0.08 sec)
MySQL (root@localhost:test 03:26:20) >show warnings;
+---------+------+---------------------------------------------+
| Level | Code | message |
+---------+------+---------------------------------------------+
| Warning | 1264 | Out of range value for column ' ID ' at row 1 |
| Warning | 1265 | Data truncated for column ' name ' at row 1 |
+---------+------+---------------------------------------------+
2 rows in Set (0.00 sec)
MySQL (root@localhost:test 03:26:27) >select * from C;
+------+------+
| ID | name |
+------+------+
| 127 | C |
+------+------+
1 row in Set (0.02 sec)
MySQL (root@localhost:test 03:26:40) >insert into C values ("The", ' Chen ');
Query OK, 1 row affected, 2 warnings (0.05 sec)
MySQL (root@localhost:test 03:26:53) >select * from C;
+------+------+
| ID | name |
+------+------+
| 127 | C |
| 127 | C |
+------+------+
2 rows in Set (0.00 sec)
The tinyint here is a byte, an integer that can represent the range from 0-255, but why not here until 127, The
reason is that we didn't give him an unsigned type.
