Check out example codes for "alter table mysql". It will help you in understanding the concepts better.

Code Example 1

ALTER TABLE table_name

MODIFY column_name datatype;

Code Example 2

ALTER TABLE contacts
  ADD last_name varchar(40) NOT NULL
    AFTER contact_id,
  ADD first_name varchar(35) NULL
    AFTER last_name;

Code Example 3

ALTER TABLE `TABLE_NAME` 
	ADD `COLUMN_NAME` VARCHAR(50) NULL
    AFTER `COLUMN_NAME_AFTER`;

Code Example 4

ALTER TABLE table
ADD COLUMN column VARCHAR (255) NOT NULL AFTER column;

Code Example 5

ALTER TABLE table
  ADD [COLUMN] column_name column_definition
  [FIRST|AFTER existing_column];

Code Example 6

ALTER TABLE nom_table
MODIFY nom_colonne type_donnees

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