SQL ALTER TABLE Statement
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 our complete 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 |
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