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默认约束(DEFAULT)
默认值
当插入记录时,如果没有明确为字段赋值,则自动赋予默认值。
mysql> CREATE TABLE tb8(
-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-> usename VARCHAR(20) NOT NULL UNIQUE KEY,
-> sex ENUM(‘1‘,‘2‘,‘3‘) DEFAULT ‘3‘
-> );
Query OK, 0 rows affected (0.11 sec)
mysql> SHOW COLUMNS FROM tb8;
+---------+----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+----------------------+------+-----+---------+----------------+
| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment|
| usename | varchar(20) | NO | UNI | NULL | |
| sex | enum(‘1‘,‘2‘,‘3‘) | YES | | 3 | |
+---------+----------------------+------+-----+---------+----------------+
3 rows in set (0.03 sec)
mysql> INSERT tb8(usename) VALUES(‘Tom‘); #插入记录
Query OK, 1 row affected (0.03 sec)
mysql> SELECT * FROM tb8; #查看记录
+----+---------+------+
| id | usename| sex |
+----+---------+------+
| 1 | Tom | 3 |
+----+---------+------+
1 row in set (0.00 sec)
默认约束(DEFAULT)