14、MySQL-唯一約束(UNIQUE KEY)
mysql> CREATE TABLE IF NOT EXISTS testUNIQUE(
-> id INT AUTO_INCREMENT KEY,
-> username VARCHAR(20) UNIQUE
-> ); mysql> INSERT testUNIQUE(username) VALUES('AAA');
mysql> INSERT testUNIQUE(username) VALUES('AAA'); -- 記錄重複時,會提示錯誤
ERROR 1062 (23000): Duplicate entry 'AAA' for key 'username'
mysql> INSERT testUNIQUE(username) VALUES(NULL);
mysql> INSERT testUNIQUE(username) VALUES(NULL); -- 記錄重複NULL時,不會提示錯誤 mysql> SELECT * FROM testUNIQUE;
+----+----------+
| id | username |
+----+----------+
| 3 | NULL |
| 4 | NULL |
| 1 | AAA |
+----+----------+