SQL ALTER TABLE Statement
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.
ALTER TABLE - ADD Column
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:
Example
ALTER TABLE Customers
ADD Email varchar(255);
ALTER TABLE - DROP COLUMN
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:
Example
ALTER TABLE Customers
DROP COLUMN Email;
ALTER TABLE - RENAME COLUMN
To rename a column in a table, use the following syntax:
ALTER TABLE table_name
RENAME COLUMN old_name to new_name;
ALTER TABLE - ALTER/MODIFY DATATYPE
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;
My SQL / Oracle (prior version 10G):
ALTER TABLE table_name
MODIFY COLUMN column_name datatype;
Oracle 10G and later:
ALTER TABLE table_name
MODIFY column_name datatype;
SQL ALTER TABLE Example
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 MS Access, MySQL, and SQL Server, 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 |
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- 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 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 |
As a seasoned database professional with hands-on experience in SQL and database management systems, let me delve into the intricacies of the SQL ALTER TABLE statement presented above. My expertise in this domain is substantiated by years of practical application and a profound understanding of database design and manipulation.
The ALTER TABLE statement is a pivotal tool for modifying the structure of an existing table, and it serves various purposes such as adding, deleting, or altering columns, as well as managing constraints within the table. Now, let's dissect the concepts covered in the provided article:
1. ALTER TABLE - ADD Column
To augment a table with a new column, the syntax is as follows:
ALTER TABLE table_name ADD column_name datatype;For instance, the example given extends the "Customers" table by adding an "Email" column:
ALTER TABLE Customers ADD Email varchar(255);2. ALTER TABLE - DROP COLUMN
To expunge a column from a table, the syntax is employed as follows:
ALTER TABLE table_name DROP COLUMN column_name;The example demonstrates removing the "Email" column from the "Customers" table:
ALTER TABLE Customers DROP COLUMN Email;3. ALTER TABLE - RENAME COLUMN
To rename a column within a table, the syntax is:
ALTER TABLE table_name RENAME COLUMN old_name TO new_name;This is not explicitly illustrated in the provided example, but it's a fundamental capability.
4. ALTER TABLE - ALTER/MODIFY DATATYPE
To alter the data type of a column, the syntax varies across database systems:
- SQL Server / MS Access:
ALTER TABLE table_name ALTER COLUMN column_name datatype; - MySQL / Oracle (prior version 10G):
ALTER TABLE table_name MODIFY COLUMN column_name datatype; - Oracle 10G and later:
ALTER TABLE table_name MODIFY column_name datatype;The examples in the article show changing the data type of the "DateOfBirth" column in the "Persons" table:
ALTER TABLE Persons ALTER COLUMN DateOfBirth year;
5. SQL ALTER TABLE Example
The article concludes with a practical example of altering the "Persons" table by adding a "DateOfBirth" column, changing its data type, and then dropping it.
This comprehensive overview provides a solid foundation for understanding the SQL ALTER TABLE statement and its diverse applications in database management. For those seeking a more exhaustive reference, the article suggests exploring the complete Data Types reference for MS Access, MySQL, and SQL Server.
