Description

The ALTER TABLE statement can be used to alter or modify an existing table.

  • It can be used to add, modify, or delete table columns.
  • It can be used to add and drop table constraints.
  • It can be used to rename a table.

After creating a table, we may get into a situation where we need to modify the table definition of a table that is in use. In such situations, the ALTER TABLE statement comes handy to do such changes on an existing table without corrupting the table data.

Let's look at an example of the ALTER TABLE statement to modify an existing column data type.

Table Data

Before executing the query, consider the below table data.

customer_id customer_name address city country postal_code
1 Maria Anders Obere Str. 57 Berlin Germany 12209
2 Yoshi Nagase 9-8 Sekimai Musashino-shi Tokyo Japan 100
3 Dominique Perrier 25, rue Lauriston Paris France 75016
4 Martin Blank Via Monte Bianco 34 Turin Italy 10100
5 Thomas Hardy 89 Chiaroscuro Rd. Portland USA 97219

Example

The below SQL ALTER TABLE statement modifies an existing column on the table, by changing its data type.

Run this on IDE

ALTER TABLE customers
MODIFY email VARCHAR(50);

After successful execution, the table data looks as below.

customer_id customer_name address city country postal_code email
1 Maria Anders Obere Str. 57 Berlin Germany 12209 NULL
2 Yoshi Nagase 9-8 Sekimai Musashino-shi Tokyo Japan 100 NULL
3 Dominique Perrier 25, rue Lauriston Paris France 75016 NULL
4 Martin Blank Via Monte Bianco 34 Turin Italy 10100 NULL
5 Thomas Hardy 89 Chiaroscuro Rd. Portland USA 97219 NULL

Overall

We now know how to use ALTER TABLE statement to modify an existing column data type.

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