The ALTER TABLE Statement The ALTER TABLE statement is used to add, delete, or modify columns in an existing table. SQL ALTER TABLE Syntax ...
The ALTER TABLE Statement
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 table_name 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 column_name |
To change the data type of a column in a table, use the following syntax:
ALTER TABLE table_name ALTER COLUMN column_name datatype |
SQL ALTER TABLE Example
Look at the "Persons" table:| P_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 MS Access, MySQL, and SQL Server, go to Data Types reference.
The "Persons" table will now like this:
| P_Id | LastName | FirstName | Address | City | DateOfBirth |
|---|---|---|---|---|---|
| 1 | Hansen | Ola | Timoteivn 10 | Sandnes | |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes | |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
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 | Hansen | Ola | Timoteivn 10 | Sandnes |
| 2 | Svendson | Tove | Borgvn 23 | Sandnes |
| 3 | Pettersen | Kari | Storgt 20 | Stavanger |
