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SQL: Alter Table

How to use the alter table SQL statement to modify existing database tables.

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Edited: 2020-07-25 01:12

Alter table, SQL

The alter table statement is used to change the composition of a table; a common usage is to change column names and data types, but it also allows adding and dropping columns to a table.

The below query will change the datatype of the test column in the my_test_table table:

alter table my_test_table modify column test varchar(200);

Adding and removing columns

To add a new column to an existing table:

alter table my_table_name add name_of_new_column varchar(255);

To remove a column from an existing table:

alter table my_table_name drop column name_of_column;

Changing the datatype or the length of a column

Sometimes we might need to change the datatype or length of an existing column, to do that we should use the modify column statement:

alter table my_test_table modify column test varchar(200);

We should carefully choose a datatype and length that suits the needs of our application.

Note. The maximum size of a varchar column is 65.535 characters.

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