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SQL Quiz
SQL QuizSQL 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
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
DROP COLUMN column_name
To change the data type of a column in a table, use the following syntax:
SQL Server / MS Access:
ALTER TABLE table_name
ALTER COLUMN column_name datatype
ALTER COLUMN column_name datatype
My SQL / Oracle:
ALTER TABLE table_name
MODIFY column_name datatype
MODIFY 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
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
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
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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