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The ALTER TABLE statement is used to add, delete, or modify columns in an existing table
The ALTER TABLE
statement is used to add, delete, or modify columns in an existing table.
The ALTER TABLE
statement is also used to add and drop various constraints on
an existing table.
To add a column in a table, use the following syntax:
ALTER TABLE table_name
ADD column_name datatype;
The following SQL adds an "Email" column to the "Customers" table:
ALTER TABLE Customers
ADD Email varchar(255);
To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):
ALTER TABLE table_name
DROP COLUMN column_name;
The following SQL deletes the "Email" column from the "Customers" table:
ALTER TABLE Customers
DROP COLUMN Email;
To change the data type of a column in a table, use the following syntax:
ALTER TABLE table_name
MODIFY COLUMN column_name datatype;
Look at the "Persons" table:
ID | LastName | FirstName | Address | City |
---|---|---|---|---|
1 | Hansen | Ola | Timoteivn 10 | Sandnes |
2 | Svendson | Tove | Borgvn 23 | Sandnes |
3 | Pettersen | Kari | Storgt 20 | Stavanger |
Now we want to add a column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
ADD DateOfBirth date;
Notice that the new column, "DateOfBirth", is of type date and is going to hold a date. The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MySQL, go to our complete Data Types reference.
The "Persons" table will now look like this:
ID | LastName | FirstName | Address | City | DateOfBirth |
---|---|---|---|---|---|
1 | Hansen | Ola | Timoteivn 10 | Sandnes | |
2 | Svendson | Tove | Borgvn 23 | Sandnes | |
3 | Pettersen | Kari | Storgt 20 | Stavanger |
Now we want to change the data type of the column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
MODIFY COLUMN DateOfBirth year;
Notice that the "DateOfBirth" column is now of type year and is going to hold a year in a two- or four-digit format.
Next, we want to delete the column named "DateOfBirth" in the "Persons" table.
We use the following SQL statement:
ALTER TABLE Persons
DROP COLUMN DateOfBirth;
The "Persons" table will now look like this:
ID | LastName | FirstName | Address | City |
---|---|---|---|---|
1 | Hansen | Ola | Timoteivn 10 | Sandnes |
2 | Svendson | Tove | Borgvn 23 | Sandnes |
3 | Pettersen | Kari | Storgt 20 | Stavanger |