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The MySQL Workshop
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In addition to creating tables, it is also possible to modify existing tables. This can be done using an ALTER
query. An ALTER
query uses the following syntax:
ALTER TABLE [table_name] [alter_options]
ALTER
queries can be used for a number of purposes. One common reason is to change how a field in a table is defined. For example, suppose we have a customer table that contains a field for username. Currently, it allows for a VARCHAR
value of size 15
, but we want to extend this to be size 30
. To do this, we can use an ALTER
query, as follows:
ALTER TABLE customer MODIFY username VARCHAR(30);
We can also use the ALTER
query to add an index to our table. To do this, we first need to create the index using a CREATE
query. So, for example, suppose we now wanted to add an index to the username of our customer table. First, we create the index for username
:
CREATE UNIQUE INDEX 'idx_username' ON customer('username')
The next exercise...