Altering Database Tables
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/*
mysql> CREATE TABLE Employee (
-> Name VARCHAR(50) PRIMARY KEY NOT NULL,
-> PhoneNo VARCHAR(15) DEFAULT 'Unknown Phone',
-> Age INT CHECK (Age BETWEEN 20 and 30));
Query OK, 0 rows affected (0.14 sec)
mysql> Describe Employee;
+---------+-------------+------+-----+---------------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------------+-------+
| Name | varchar(50) | | PRI | | |
| PhoneNo | varchar(15) | YES | | Unknown Phone | |
| Age | int(11) | YES | | NULL | |
+---------+-------------+------+-----+---------------+-------+
3 rows in set (0.01 sec)
mysql> Select * from Employee;
+----------+---------------+------+
| Name | PhoneNo | Age |
+----------+---------------+------+
| John Doe | Unknown Phone | 31 |
+----------+---------------+------+
1 row in set (0.00 sec)
mysql> ALTER TABLE Employee DROP COLUMN PhoneNo;
Query OK, 1 row affected (0.05 sec)
Records: 1 Duplicates: 0 Warnings: 0
mysql> Describe Employee;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| Name | varchar(50) | | PRI | | |
| Age | int(11) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
mysql> Select * from Employee;
+----------+------+
| Name | Age |
+----------+------+
| John Doe | 31 |
+----------+------+
1 row in set (0.00 sec)
*/
Drop TABLE Employee;
CREATE TABLE Employee (
Name VARCHAR(50) PRIMARY KEY NOT NULL,
PhoneNo VARCHAR(15) DEFAULT 'Unknown Phone',
Age INT CHECK (Age BETWEEN 20 and 30));
Describe Employee;
INSERT INTO Employee (Name, Phone, Age) VALUES ('Joe Yin', '666 2323', 26);
INSERT INTO Employee (Name, Age) VALUES ('John Doe', 31);
Select * from Employee;
ALTER TABLE Employee DROP COLUMN PhoneNo;
Describe Employee;
Select * from Employee;
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