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ALTER

The ALTER statement is used to modify an existing table in SQL. The syntax for altering a table is as follows:

ALTER TABLE table_name
    ADD column_name datatype constraints,
    MODIFY column_name datatype constraints,
    DROP COLUMN column_name;

flowchart TD
    A["ALTER TABLE table_name"] --> B["Choose Action"]
    B --> C["ADD column_name datatype constraints"]
    B --> D["MODIFY column_name datatype constraints"]
    B --> E["DROP COLUMN column_name"]

    C --> F["Table Updated"]
    D --> F
    E --> F
  • table_name: The name of the table you want to modify.
  • ADD column_name datatype constraints: Adds a new column to the table.
  • MODIFY column_name datatype constraints: Modifies the data type or constraints of an existing column.
  • DROP COLUMN column_name: Removes a column from the table.

Example:

ALTER TABLE employees
    ADD email VARCHAR(255),
    MODIFY salary DECIMAL(12, 2),
    DROP COLUMN position;

erDiagram
    EMPLOYEES {
        INT id PK
        VARCHAR name
        VARCHAR position
        DECIMAL salary "10,2"
    }
 
erDiagram
    EMPLOYEES {
        INT id PK
        VARCHAR name
        VARCHAR email
        DECIMAL salary "12,2"
    }