MySQL - ALTER TABLE Statement

MySQL ALTER TABLE Statement

The Alter TABLE statement of MySQL helps to perform changes on a table. Using this statement, we can add, delete, or modify columns in an existing table and we can also rename it.

Syntax

Following is the syntax of the ALTER Table statement −

ALTER TABLE table_name [alter_option ...];

Where, table_name is the name of the existing table you need to change and the alter_option represents the change you need to perform on the table such as ADD COLUMNS, DROP COLUMN etc.

Adding a new column

For instance, if you need to add a new column to a table you should use the ADD COLUMN option as shown below −

ALTER TABLE table_name ADD COLUMN (column_name column_definition...)

Example

Suppose we have created a table as shown below −

CREATE TABLE Employee(
   Name VARCHAR(255),
   Salary INT NOT NULL,
   Location VARCHAR(255)
);

Following query adds a new column named phone to the Employee table −

ALTER TABLE Employee ADD COLUMN (Phone int (10));

To verify the above query if you describe the table using the DESC command you can observe the created column in the field list as shown below −

DESC employee;

Following is the output of the above mysql query −

Adding a column after an existing column

If you want to add a column before or after an existing column you need to use AFTER followed by the name of the existing column (after the column definition).

ALTER TABLE table_name ADD COLUMN (column_name column_definition...)

Example

Following query adds a column named address after the Location.

ALTER TABLE Employee ADD COLUMN Address VARCHAR(50) AFTER Location;

To verify the above query if you describe the table using the DESC command you can observe the created column in the field list as shown below −

DESC employee;

Output

The above query produces the following output &mius;

Adding a column at start

You can add the new column at the start using the FIRST statement along with alter, following is the syntax to do so −

ALTER TABLE table_name ADD COLUMN (column_name column_definition...)

Example

Following query adds a column with name ID at start −

ALTER TABLE Employee ADD COLUMN ID INT FIRST;

To verify the above query if you describe the table using the DESC command you can observe the created column in the field list as shown below −

desc employee;

Output

Following is the output of the above query −

Dropping a column

You can drop an existing column by using the DROP statement along with ALTER following is the syntax to do so −

ALTER TABLE table_name DROP COLUMN column_name;

Example

First of all, let us verify the definition of the table using DESC statement −

desc employee;

Output

The above query produces the following output −

Following query drops the column named ID −

ALTER TABLE Employee DROP COLUMN ID;

To verify the above query if you describe the table using the DESC command you can observe the name of the deleted column is missing from the field list −

desc employee;

Following is the output of the above query −

Adding an index

You can add index to an existing column of a table using the ADD INDEX statement along with the ALTER statement −

ALTER TABLE table_name ADD INDEX index_name [index_type] (key_part,...)

Example

Following query adds an index on the columns Name and Salary −

ALTER TABLE Employee ADD INDEX sample_index (Name, Salary);

You can verify the indices of the table using the SHOW INDEX statement as shown below −

SHOW INDEX FROM Employee;

Output

The above query will produce the following output −

*************** 1. row ***************
        Table: employee
   Non_unique: 1
     Key_name: sample_index
 Seq_in_index: 1
  Column_name: Name
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment:
Index_comment:
      Visible: YES
   Expression: NULL
*************** 2. row ***************
        Table: employee
   Non_unique: 1
     Key_name: sample_index
 Seq_in_index: 2
  Column_name: Salary
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
      Visible: YES
   Expression: NULL

Adding a primary key

Following is the syntax of creating a primary key −

ALTER TABLE table_name ADD CONSTRAINT MyPrimaryKey 
PRIMARY KEY (column1, column2...);

Example

Let us create a column named ID in the table Employee −

ALTER TABLE Employee ADD COLUMN ID INT FIRST;

Following query creates a primary key on the column ID −

ALTER TABLE Employee ADD CONSTRAINT MyPrimaryKey PRIMARY KEY(ID);

To verify the above query if you describe the table using the DESC command you can observe PRI against the ID field under the column Key −

desc employee;

Output

Following is the output of the above program −

Adding a foreign key

Following is the syntax of creating a foreign key −

ALTER TABLE table_name ADD CONSTRAINT key FORIEGN KEY (column_name);

Example

Assume we have created another table named test as shown below −

CREATE table Test (ID int PRIMARY KEY);

Let us create a column named ID in the table Employee −

ALTER TABLE Employee ADD COLUMN ID INT FIRST;

Following query creates a primary key on the column ID −

ALTER TABLE Employee ADD CONSTRAINT fk FOREIGN KEY(ID) REFERENCES test(ID);

Adding a constraint

Following is the syntax to add constraint to a column −

ALTER TABLE table_name ADD CONSTRAINT constraint_name 
UNIQUE(column1, column2...);

Example

Following query adds UNIQUE constraint to the table Employee −

ALTER TABLE Employee ADD CONSTRAINT con UNIQUE(Phone);

Dropping an index

Following is the syntax to drop an index on a table −

ALTER TABLE table_name DROP INDEX index_name

Example

Following query drops the index created above on the table Employee −

ALTER TABLE Employee DROP INDEX sample_index;

Drop a primary key

Following is the syntax to drop a primary key −

ALTER TABLE table_name DROP PRIMARY key_name;

Example

Suppose we have created a table and added a PRIMARY KEY as shown below −

create table sample (ID INT);
alter table sample add CONSTRAINT PRIMARY KEY (ID);

To verify the above query if you describe the table using the DESC command you can observe PRI against the ID field under the column Key −

desc sample;

Output

Following is the output of the above query −

Following query drops the above created Primary key −

alter table sample drop PRIMARY KEY;

If you describe the sample table using the desc statement you can observe that the primary constraint is removed −

desc sample;

Output

The above query produces the following output −

Dropping a foreign key

Following is the syntax to drop a foreign key −

ALTER TABLE table_name DROP PRIMARY key_name;

Example

Following query drops the foreign key fk of the table employee −

ALTER TABLE Employee DROP FOREIGN KEY fk;