Setting a Unique Constraint : Unique « Select Clause « SQL / MySQL

1. SQL / MySQL
2. Select Clause
3. Unique

Setting a Unique Constraint

/*
mysql> Drop TABLE Employee;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE Employee (
    ->    Name    VARCHAR(50) NOT NULL,
    ->    PhoneNo VARCHAR(15) DEFAULT 'Unknown Phone' NOT NULL,
    ->    CONSTRAINT MyUniqueKey UNIQUE (Name));
Query OK, 0 rows affected (0.07 sec)

mysql> Describe Employee;
+---------+-------------+------+-----+---------------+-------+
| Field   | Type        | Null | Key | Default       | Extra |
+---------+-------------+------+-----+---------------+-------+
| Name    | varchar(50) |      | PRI |               |       |
| PhoneNo | varchar(15) |      |     | Unknown Phone |       |
+---------+-------------+------+-----+---------------+-------+
2 rows in set (0.00 sec)

mysql> INSERT INTO Employee (Name, Phone) VALUES ('Joe Wang', '666 2323');
ERROR 1054 (42S22): Unknown column 'Phone' in 'field list'
mysql> INSERT INTO Employee (Name) VALUES ('John Doe');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO Employee (Name, Phone) VALUES ('Joe Wang', NULL);
ERROR 1054 (42S22): Unknown column 'Phone' in 'field list'
mysql> Select * from Employee;
+----------+---------------+
| Name     | PhoneNo       |
+----------+---------------+
| John Doe | Unknown Phone |
+----------+---------------+
1 row in set (0.00 sec)


*/
Drop TABLE Employee;       
       
CREATE TABLE Employee (
   Name    VARCHAR(50) NOT NULL, 
   PhoneNo VARCHAR(15) DEFAULT 'Unknown Phone' NOT NULL,
   CONSTRAINT MyUniqueKey UNIQUE (Name));

Describe Employee;

INSERT INTO Employee (Name, Phone) VALUES ('Joe Wang', '666 2323');
INSERT INTO Employee (Name) VALUES ('John Doe');
INSERT INTO Employee (Name, Phone) VALUES ('Joe Wang', NULL);

Select * from Employee;

           
       

Related examples in the same category