Oracle ALTER TABLE ADD Column By Examples

Summary: in this tutorial, you will learn how to use the Oracle ALTER TABLE ADD column statement to add one or more columns to a table.

To add a new column to a table, you use the ALTER TABLE statement as follows:

ALTER TABLE table_name 
ADD column_name data_type constraint;

In this statement:

• First, you specify the name of the table, which you want to add the new column, after the ALTER TABLE clause.
• Second, you specify the column name, data type, and its constraint.

Note that you cannot add a column that already exists in the table; trying to do so will cause an error. In addition, the ALTER TABLE ADD column statement adds the new column at the end of the table. Oracle provides no direct way to allow you to specify the position of the new column like other database systems such as MySQL.

In case you want to add more than one column, you use the following syntax:

ALTER TABLE table_name 
ADD (
    column_name_1 data_type constraint,
    column_name_2 data_type constraint,
    ...
);

In this syntax, you separate two columns by a comma.

Oracle ALTER TABLE ADD column examples

Let’s create a table named members for the demonstration.

CREATE TABLE members(
    member_id NUMBER GENERATED BY DEFAULT AS IDENTITY,
    first_name VARCHAR2(50),
    last_name VARCHAR2(50),
    PRIMARY KEY(member_id)
);

The following statement adds a new column named birth_date to the members table:

ALTER TABLE members 
ADD birth_date DATE NOT NULL;

In this example, the birth_date column is a DATE column and it does not accept null.

Suppose, you want to record the time at which a row is created and updated. To do so, you need to add two columns created_at and updated_at as follows:

ALTER TABLE
    members ADD(
        created_at TIMESTAMP WITH TIME ZONE NOT NULL,
        updated_at TIMESTAMP WITH TIME ZONE NOT NULL
    );

The data types of the created_at and updated_at columns are TIMESTAMP WITH TIME ZONE. These columns also do not accept null.

To check whether a column exists in a table, you query the data from the user_tab_cols view. For example, the following statement checks whether the members table has the first_name column.

SELECT
    COUNT(*)
FROM
    user_tab_cols
WHERE
    column_name = 'FIRST_NAME'
    AND table_name = 'MEMBERS';

This query comes in handy when you want to check whether a column exists in a table before adding it.

For example, the following PL/SQL block checks whether the members table has effective_date column before adding it.

SET SERVEROUTPUT ON SIZE 1000000
DECLARE
  v_col_exists NUMBER 
BEGIN
  SELECT count(*) INTO v_col_exists
    FROM user_tab_cols
    WHERE column_name = 'EFFECTIVE_DATE'
      AND table_name = 'MEMBERS';

   IF (v_col_exists = 0) THEN
      EXECUTE IMMEDIATE 'ALTER TABLE members ADD effective_date DATE';
   ELSE
    DBMS_OUTPUT.PUT_LINE('The column effective_date already exists');
  END IF;
END;
/

If you execute the block at the first time, the effective_date column is appended at the end of the members table. However, once you execute it from the second time, you will see the following message:

The column effective_date already exists

which is what we expected.

In this tutorial, you have learned how to use the Oracle ALTER TABLE ADD column statement to add one or more column to an existing table.