After you’ve spent time designing the perfect table, someone is going to come along and ask you a question that will lead you to changing your table. It can be something as simple as, hey, we can only store 25 characters for a city name, we need to have 50. Or it can be a little more complex, like adding or dropping a column. It can also be the worst case scenario, where you’re asked to re-order the columns.
By the way, that requires you backing up the table to a separate table, dropping the original table, creating a new table with the updated order, then select all the data back into that table. And don’t forget to drop your backup table when you’re finished!
Making changes to tables is pretty easy, you just need the ALTER TABLE command. The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.
SQL ALTER TABLE Syntax
To add a column in a table, use the following syntax:
ALTER TABLE tableName ADD column_name datatype
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 columnName
To change the data type of a column in a table, use the following syntax:
ALTER TABLE tableName ALTER COLUMN columnName datatype
SQL ALTER TABLE Example
Look at the “Persons” table:
| P_Id | LastName | FirstName | Address | City |
|---|---|---|---|---|
| 1 | Skywalker | Luke | 1 Dry-Ass Desert Way | Greater Mos Eisley |
| 2 | Lars | Owen | 1 Dry-Ass Desert Way | Greater Mos Eisley |
| 3 | Ackbar | Admiral | 223 Great Mon Calamari Drive | Mon Calamari City |
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.
The “Persons” table will now like this:
| P_Id | LastName | FirstName | Address | City | DateOfBirth |
|---|---|---|---|---|---|
| 1 | Skywalker | Luke | 1 Dry-Ass Desert Way | Greater Mos Eisley | |
| 2 | Lars | Owen | 1 Dry-Ass Desert Way | Greater Mos Eisley | |
| 3 | Ackbar | Admiral | 223 Great Mon Calamari Drive | Mon Calamari City |
Change Data Type Example
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 ALTER COLUMN DateOfBirth year
Notice that the “DateOfBirth” column is now of type year and is going to hold a year in a two-digit or four-digit format.
DROP COLUMN Example
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 like this:
| P_Id | LastName | FirstName | Address | City |
|---|---|---|---|---|
| 1 | Skywalker | Luke | 1 Dry-Ass Desert Way | Greater Mos Eisley |
| 2 | Lars | Owen | 1 Dry-Ass Desert Way | Greater Mos Eisley |
| 3 | Ackbar | Admiral | 223 Great Mon Calamari Drive | Mon Calamari City |
If you have any questions, send them in! I’m here to help!