SQL ALTER TABLE Statement

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SQL ALTER TABLE  Statement
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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;
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;
ALTER TABLE - ALTER/MODIFY COLUMN
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

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