MySQL SQL to add new fields to the table implementation method

If you want to add columns to the table, use the following syntax:

ALTER TABLE table_name
ADD column_name datatype
To delete a column from a table, use the following syntax:

ALTER TABLE table_name
DROP COLUMN column_name
Note: Some database tutorial systems do not allow this way to delete columns in a database table (DROP column column_name)

。

Instance
MySQL Tutorial > CREATE TABLE Employee (
   ->     id             int,
   ->     first_name    VARCHAR,
   ->     last_name      VARCHAR,
   ->     start_date    Date,
   ->     end_date      date,
    ->     salary        FLOAT (8,2),
    ->     city          VARCHAR (10) ,
   ->     description   VARCHAR
   - >);
Query OK, 0 rows affected (0.02 sec)

Mysql>
Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> values (1, ' Jason ', ' Martin ', ' 19960725 ', ' 20060725 ',

1234.56, ' Toronto ', ' Programmer ');
Query OK, 1 row Affected (0.00 sec)

Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> values (2, ' Alison ', ' Mathews ', ' 19760321 ', ' 19860221 ',

6661.78, ' Vancouver ', ' Tester ');
Query OK, 1 row Affected (0.00 sec)

Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> values (3, ' James ', ' Smith ', ' 19781212 ', ' 19900315 ',

6544.78, ' Vancouver ', ' Tester ');
Query OK, 1 row Affected (0.00 sec)

Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> VALUES (4, ' Celia ', ' Rice ', ' 19821024 ', ' 19990421 ',

2344.78, ' Vancouver ', ' Manager ';
Query OK, 1 row Affected (0.00 sec)

Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> values (5, ' Robert ', ' black ', ' 19840115 ', ' 19980808 ',

2334.78, ' Vancouver ', ' Tester ');
Query OK, 1 row Affected (0.00 sec)

Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> VALUES (6, ' Linda ', ' Green ', ' 19870730 ', ' 19960104 ',

4322.78, ' New York ', ' Tester ';
Query OK, 1 row Affected (0.00 sec)

Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> VALUES (7, ' David ', ' Larry ', ' 19901231 ', ' 19980212 ',

7897.78, ' New York ', ' Manager ';
Query OK, 1 row Affected (0.00 sec)

Mysql>
mysql> INSERT INTO Employee (Id,first_name, last_name, start_date, End_date,

Salary, city, Description)
-> VALUES (8, ' James ', ' Cat ', ' 19960917 ', ' 20020415 ',

1232.78, ' Vancouver ', ' Tester ');
Query OK, 1 row Affected (0.00 sec)

Mysql>
Mysql> select * from Employee;
+------+------------+-----------+------------+------------+---------+-----------

+-------------+
| ID | first_name | last_name | start_date | end_date | Salary | City |

Description |
+------+------------+-----------+------------+------------+---------+-----------

+-------------+
| 1 | Jason | Martin | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto |

Programmer |
| 2 | Alison | Mathews | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver |

Tester |
| 3 | James | Smith | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver |

Tester |
| 4 | Celia | Rice | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver |

Manager |
| 5 | Robert | Black | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver |

Tester |
| 6 | Linda | Green | 1987-07-30 | 1996-01-04 | 4322.78 | New York |

Tester |
| 9 2 David | Larry | 1990-12-31 | 1998-02-12 | 7897.78 | New York |

Manager |
| 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver |

Tester |
+------+------------+-----------+------------+------------+---------+-----------

+-------------+
8 rows in Set (0.00 sec)

Mysql>
Mysql>
Mysql>
Mysql> DESC employee;
+-------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| ID | Int (11) |     YES | |       NULL | |
| first_name | varchar (15) |     YES | |       NULL | |
| last_name | varchar (15) |     YES | |       NULL | |
| start_date | Date |     YES | |       NULL | |
| end_date | Date |     YES | |       NULL | |
| Salary | Float (8,2) |     YES | |       NULL | |
| City | varchar (10) |     YES | |       NULL | |
| Description | varchar (15) |     YES | |       NULL | |
+-------------+-------------+------+-----+---------+-------+
8 rows in Set (0.03 sec)

Mysql>
Mysql> ALTER TABLE Employee
-> ADD new_date Date;
Query OK, 8 rows affected (0.16 sec)
Records:8 duplicates:0 warnings:0

Mysql>
Mysql> Describe employee;
+-------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| ID | Int (11) |     YES | |       NULL | |
| first_name | varchar (15) |     YES | |       NULL | |
| last_name | varchar (15) |     YES | |       NULL | |
| start_date | Date |     YES | |       NULL | |
| end_date | Date |     YES | |       NULL | |
| Salary | Float (8,2) |     YES | |       NULL | |
| City | varchar (10) |     YES | |       NULL | |
| Description | varchar (15) |     YES | |       NULL | |
| New_date | Date |     YES | |       NULL | |
+-------------+-------------+------+-----+---------+-------+
9 rows in Set (0.06 sec)

Mysql>
Mysql> SELECT ID, new_date from employee;
+------+----------+
| ID | New_date |
+------+----------+
| 1 | NULL |
| 2 | NULL |
| 3 | NULL |
| 4 | NULL |
| 5 | NULL |
| 6 | NULL |
| 7 | NULL |
| 8 | NULL |
+------+----------+
8 rows in Set (0.00 sec)

Example two, using the above table

Mysql> ALTER TABLE Employee ADD middle_name VARCHAR (30);
Query OK, 8 rows Affected (0.02 sec)
Records:8 duplicates:0 warnings:0

Mysql>
Mysql> select * from Employee;
+------+------------+-----------+------------+------------+---------+-----------

+-------------+-------------+
| ID | first_name | last_name | start_date | end_date | Salary | City |

Description | Middle_name |
+------+------------+-----------+------------+------------+---------+-----------

+-------------+-------------+
| 1 | Jason | Martin | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto |

Programmer | NULL |
| 2 | Alison | Mathews | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver |

Tester | NULL |
| 3 | James | Smith | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver |

Tester | NULL |
| 4 | Celia | Rice | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver |

Manager | NULL |
| 5 | Robert | Black | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver |

Tester | NULL |
| 6 | Linda | Green | 1987-07-30 | 1996-01-04 | 4322.78 | New York |

Tester | NULL |
| 9 2 David | Larry | 1990-12-31 | 1998-02-12 | 7897.78 | New York |

Manager | NULL |
| 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver |

Tester | NULL |
+------+------------+-----------+------------+------------+---------+-----------

+-------------+-------------+
8 rows in Set (0.00 sec)

Mysql>
Mysql> DESC employee;
+-------------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| ID | Int (11) |     YES | |       NULL | |
| first_name | varchar (15) |     YES | |       NULL | |
| last_name | varchar (15) |     YES | |       NULL | |
| start_date | Date |     YES | |       NULL | |
| end_date | Date |     YES | |       NULL | |
| Salary | Float (8,2) |     YES | |       NULL | |
| City | varchar (10) |     YES | |       NULL | |
| Description | varchar (15) |     YES | |       NULL | |
| Middle_name | varchar (30) |     YES | |       NULL | |
+-------------+-------------+------+-----+---------+-------+
9 Rows in Set (0.02 sec)