> This page location: Managing Tables > Alter Table
> Full Neon documentation index: https://neon.com/docs/llms.txt

# PostgreSQL ALTER TABLE

**Summary**: in this tutorial, you will learn how to use the PostgreSQL `ALTER TABLE` statement to modify the structure of a table.

## Introduction to PostgreSQL ALTER TABLE statement

To change the structure of an existing table, you use PostgreSQL `ALTER TABLE` statement.

The following illustrates the basic syntax of the `ALTER TABLE` statement:

```sql
ALTER TABLE table_name action;
```

PostgreSQL provides you with many actions:

- [Add a column](https://neon.com/postgresql/postgresql-tutorial/postgresql-add-column)
- [Drop a column](https://neon.com/postgresql/postgresql-tutorial/postgresql-drop-column)
- [Change the data type of a column](https://neon.com/postgresql/postgresql-tutorial/postgresql-change-column-type)
- [Rename a column](https://neon.com/postgresql/postgresql-tutorial/postgresql-rename-column)
- [Set a default value for the column](https://neon.com/postgresql/postgresql-tutorial/postgresql-default-value)
- Add a constraint to a column.
- [Rename a table](https://neon.com/postgresql/postgresql-tutorial/postgresql-rename-table)

To add a new column to a table, you use [`ALTER TABLE ADD COLUMN`](https://neon.com/postgresql/postgresql-tutorial/postgresql-add-column) statement:

```sql
ALTER TABLE table_name
ADD COLUMN column_name datatype column_constraint;
```

To drop a column from a table, you use [`ALTER TABLE DROP COLUMN`](https://neon.com/postgresql/postgresql-tutorial/postgresql-drop-column) statement:

```sql
ALTER TABLE table_name
DROP COLUMN column_name;
```

To rename a column, you use the `[ALTER TABLE RENAME COLUMN](postgresql-rename-column) TO` statement:

```sql
ALTER TABLE table_name
RENAME COLUMN column_name
TO new_column_name;
```

To change a default value of the column, you use `ALTER TABLE ALTER COLUMN SET DEFAULT` or  `DROP DEFAULT`:

```sql
ALTER TABLE table_name
ALTER COLUMN column_name
[SET DEFAULT value | DROP DEFAULT];
```

To change the [`NOT NULL` constraint](https://neon.com/postgresql/postgresql-tutorial/postgresql-not-null-constraint), you use `ALTER TABLE ALTER COLUMN` statement:

```sql
ALTER TABLE table_name
ALTER COLUMN column_name
[SET NOT NULL| DROP NOT NULL];
```

To add a `CHECK` constraint, you use `ALTER TABLE ADD CHECK` statement:

```sql
ALTER TABLE table_name
ADD CHECK expression;
```

Generally, to add a constraint to a table, you use `ALTER TABLE ADD CONSTRAINT` statement:

```sql
ALTER TABLE table_name
ADD CONSTRAINT constraint_name constraint_definition;
```

To [rename a table](https://neon.com/postgresql/postgresql-tutorial/postgresql-rename-table) you use `ALTER TABLE RENAME TO` statement:

```sql
ALTER TABLE table_name
RENAME TO new_table_name;
```

## PostgreSQL ALTER TABLE examples

Let's [create a new table](https://neon.com/postgresql/postgresql-tutorial/postgresql-create-table "PostgreSQL CREATE TABLE") called `links` for practicing with the `ALTER TABLE` statement.

```sql
DROP TABLE IF EXISTS links;

CREATE TABLE links (
   link_id serial PRIMARY KEY,
   title VARCHAR (512) NOT NULL,
   url VARCHAR (1024) NOT NULL
);
```

To [add a new column](https://neon.com/postgresql/postgresql-tutorial/postgresql-add-column) named `active`, you use the following statement:

```sql
ALTER TABLE links
ADD COLUMN active boolean;
```

The following statement removes the `active`column from the `links`table:

```sql
ALTER TABLE links
DROP COLUMN active;
```

To change the name of the `title` column to `link_title`, you use the following statement:

```sql
ALTER TABLE links
RENAME COLUMN title TO link_title;
```

The following statement adds a new column named `target`to the `links`table:

```sql
ALTER TABLE links
ADD COLUMN target VARCHAR(10);
```

To set `_blank` as the default value for the `target`column in the `links`table, you use the following statement:

```sql
ALTER TABLE links
ALTER COLUMN target
SET DEFAULT '_blank';
```

If you [insert the new row](https://neon.com/postgresql/postgresql-tutorial/postgresql-insert) into the `links` table without specifying a value for the `target` column, the `target` column will take the `_blank` as the default value. For example:

```sql
INSERT INTO links (link_title, url)
VALUES('PostgreSQL Tutorial','https://neon.com/postgresql/');
```

The following statement selects data from the `links` table:

```sql
SELECT * FROM links;
```

![PostgreSQL ALTER TABLE SET DEFAULT](https://neon.com/postgresqltutorial/PostgreSQL-ALTER-TABLE-SET-DEFAULT.png)
The following statement adds a `CHECK`condition to the `target`column so that the `target`column only accepts the following values: `_self`, `_blank`, `_parent`, and `_top`:

```sql
ALTER TABLE links
ADD CHECK (target IN ('_self', '_blank', '_parent', '_top'));
```

If you attempt to insert a new row that violates the `CHECK` constraint set for the `target`column, PostgreSQL will issue an error as shown in the following example:

```sql
INSERT INTO links(link_title,url,target)
VALUES('PostgreSQL','http://www.postgresql.org/','whatever');
```

```
ERROR:  new row for relation "links" violates check constraint "links_target_check"
DETAIL:  Failing row contains (2, PostgreSQL, http://www.postgresql.org/, whatever).DETAIL:  Failing row contains (2, PostgreSQL, http://www.postgresql.org/, whatever).
```

The following statement adds a `UNIQUE` constraint to the `url` column of the `links` table:

```sql
ALTER TABLE links
ADD CONSTRAINT unique_url UNIQUE ( url );
```

The following statement attempts to insert the url that already exists:

```sql
INSERT INTO links(link_title,url)
VALUES('PostgreSQL','https://neon.com/postgresql/');
```

It causes an error due to the unique_url constraint:

```
ERROR:  duplicate key value violates unique constraint "unique_url"
DETAIL:  Key (url)=(/postgresql/) already exists.
```

The following statement changes the name of the `links` table to `urls`:

```sql
ALTER TABLE links
RENAME TO urls;
```

In this tutorial, you have learned how to use the PostgreSQL `ALTER TABLE` statement to change the structure of an existing table.

---

## Related docs (Managing Tables)

- [PostgreSQL Data Types](https://neon.com/postgresql/postgresql-tutorial/postgresql-data-types)
- [Create Table](https://neon.com/postgresql/postgresql-tutorial/postgresql-create-table)
- [Select Into](https://neon.com/postgresql/postgresql-tutorial/postgresql-select-into)
- [Create Table As](https://neon.com/postgresql/postgresql-tutorial/postgresql-create-table-as)
- [SERIAL](https://neon.com/postgresql/postgresql-tutorial/postgresql-serial)
- [Sequences](https://neon.com/postgresql/postgresql-tutorial/postgresql-sequences)
- [Identity Column](https://neon.com/postgresql/postgresql-tutorial/postgresql-identity-column)
- [Generated Columns](https://neon.com/postgresql/postgresql-tutorial/postgresql-generated-columns)
- [Rename Table](https://neon.com/postgresql/postgresql-tutorial/postgresql-rename-table)
- [Add Column](https://neon.com/postgresql/postgresql-tutorial/postgresql-add-column)
- [Drop Column](https://neon.com/postgresql/postgresql-tutorial/postgresql-drop-column)
- [Change Column's Data Type](https://neon.com/postgresql/postgresql-tutorial/postgresql-change-column-type)
- [Rename Column](https://neon.com/postgresql/postgresql-tutorial/postgresql-rename-column)
- [Drop Table](https://neon.com/postgresql/postgresql-tutorial/postgresql-drop-table)
- [Temporary Table](https://neon.com/postgresql/postgresql-tutorial/postgresql-temporary-table)
- [Truncate Table](https://neon.com/postgresql/postgresql-tutorial/postgresql-truncate-table)
- [Copy Table](https://neon.com/postgresql/postgresql-tutorial/postgresql-copy-table)