PostgreSQL ALTER Table: ADD Column, Rename Column/Table Examples

By : Juniper Willow Juniper Willow
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The ALTER TABLE command is used to alter the structure of a PostgreSQL table. It is the command used to change the table columns or the name of the table.

Syntax

Here is the syntax for the PostgreSQL ALTER TABLE command:

ALTER TABLE table-name action;

The table-name parameter is the name of the table that you need to change.

The action parameter is the action that you need to perform, such as changing the name of a column, changing the data type of a column, etc.

Description

The ALTER TABLE command changes the definition of an existing table. It takes the following subforms:

  • ADD COLUMN: this uses similar syntax as CREATE TABLE command to add a new column to a table.
  • DROP COLUMN: for dropping a table column. The constraints and indexes imposed on the columns will also be dropped.
  • SET/DROP DEFAULT: Use for removing the default value for a column. However, the change will only apply to the subsequent INSERT statements.
  • SET/DROP NOT NULL: Changes whether a column will allow nulls or not.
  • SET STATISTICS: For setting the statistics-gathering target for each column for ANALYZE operations.
  • SET STORAGE: For setting the mode of storage for a column. This will determine where the column is held, whether inline, or in a supplementary table.
  • SET WITHOUT OIDS: Use for removing the old column of the table.
  • RENAME: for changing the table name or a column name.
  • ADD table_constraint: Use for adding a new constraint to a table It uses the same syntax as CREATE TABLE command.
  • DROP CONSTRAINT: Use for dropping a table constraint.
  • OWNER: for changing the owner of a table, sequence, index or a view to a certain user.
  • CLUSTER: for marking a table to be used for carrying out future cluster operations.

Modifying a column

A column may be modified in a number of ways. Such modifications can be done using the ALTER TABLE command. Let us discuss these:

Adding a New column

To add a new column to a PostgreSQL table, the ALTER TABLE command is used with the following syntax:

ALTER TABLE table-name
  ADD new-column-name column-definition;

The table-name is the name of the table to be modified.

The new-column-name is the name of the new column to be added.

The column-definition is the data type of the new column.

See the Book table shown below:

Adding a New column in PostgreSQL

The table has two columns, id, and name. We need to add a new column to the table and give it the name author. Just run the following command:

ALTER TABLE Book
  ADD author VARCHAR(50);

After running the above command, the Book table is now as follows:

Adding a New column in PostgreSQL

The new column was added successfully.

Renaming a Table Column

We can use the ALTER TABLE command to change the name of a column. In this case, the command is used with the following syntax:

ALTER TABLE table-name
  RENAME COLUMN old-name TO new-name;

The table-name is the name of the table whose column is to be renamed.

The old-name is the old/current name of the column.

The new-name is the new name of the column. Consider the table Book shown below:

Book:

Renaming a Table Column in PostgreSQL

We need the name of the column author to book_author. Here is the command:

ALTER TABLE Book
  RENAME COLUMN author TO book_author;

After running the command, we can view the structure of the table:

Renaming a Table Column in PostgreSQL

The column name was changed successfully.

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Setting a Default Value for a Column

We can set a default value for a column such that even when you don’t specify a value for that column during INSERT operations, the default value will be used. In this case, the ALTER TABLE command can be used with the following syntax:

ALTER TABLE table-name ALTER COLUMN column-name [SET DEFAULT value];

The table-name is the name of the table whose column is to be modified.

The column-name is the name for whose default value is to be set.

The value is the default value for the column.

Consider the Book table given below:

Setting a Default Value for a Column

We need to set a default value for the book_author column. We can run the following command:

ALTER TABLE Book ALTER COLUMN book_author SET DEFAULT 'Nicholas Samuel';

Now, let us insert a row into the table:

INSERT INTO Book (id, name)
 VALUES (6, 'PostgreSQL for Beginners');

Note that we inserted values for only two columns, id and name. However, the default value has been used for book_author column:

Setting a Default Value for a Column

Adding a Check Constraint

A check constraint helps in validating the records that are being inserted into a table. We can do this by combining the ALTER TABLE command with the ADD CHECK statement. Syntax:

ALTER TABLE table-name ADD CHECK expression;

The table-name is the name of the table to be altered.

The expression is the constraint to be imposed on the table column.

Let us modify the book_author column of the Book table so that it only accepts the values, Nicholas and Samuel:

ALTER TABLE Book ADD CHECK (book_author IN ('Nicholas', 'Samuel'));

Now, let us try to insert a value other than Nicholas or Samuel into the book_author column of the Book table:

INSERT INTO Book
VALUES(7, 'Best PostgreSQL Book', 'Gregory Bush');

The statement will return the following error:

Adding a Check Constraint

The insert operation failed because we violated the check constraint.

Renaming a Table

Here is the syntax for the ALTER TABLE command for renaming a table:

ALTER TABLE table-name
  RENAME TO new-table-name;

The table-name is the current name of the table.

The new-table-name is the new name to be assigned to the table.

For example, let us change the name of the Book table to Books:

ALTER TABLE Book
  RENAME TO Books;

Using pgAdmin

Now let’s see how these actions can be performed using pgAdmin.

Adding a New column

To accomplish the same through pgAdmin, do this:

Step 1) Login to your pgAdmin account.

Step 2)

  1. From the navigation bar on the left- Click Databases.
  2. Click Demo.

Adding a New column Using pgAdmin

Step 3) Type the query in the query editor:

ALTER TABLE Book
  ADD author VARCHAR(50);

Step 4) Click the Execute button.

Adding a New column Using pgAdmin

Step 5) To check whether the column was added, do the following:

  1. Click Databases from the left navigation.
  2. Expand Demo.
  3. Expand Schemas.
  4. Expand Public.
  5. Expand Tables.
  6. Expand book.
  7. Expand Columns.

Adding a New column Using pgAdmin

The column should have been added, as shown below:

Adding a New column Using pgAdmin

Renaming a Table Column

To accomplish the same through pgAdmin, do this:

Step 1) Login to your pgAdmin account.

Step 2)

  1. From the navigation bar on the left- Click Databases.
  2. Click Demo.

Renaming a Table Column Using pgAdmin

Step 3) Type the query in the query editor:

ALTER TABLE Book
  RENAME COLUMN author TO book_author;

Step 4) Click the Execute button.

Renaming a Table Column Using pgAdmin

Step 5) To check whether the change was successful, do the following:

  1. Click Databases from the left navigation.
  2. Expand Demo.
  3. Expand Schemas.
  4. Expand Public.
  5. Expand Tables.
  6. Expand book.
  7. Expand Columns.

Renaming a Table Column Using pgAdmin

The columns should now be as follows:

Renaming a Table Column Using pgAdmin

The column was changed successfully.

Setting a Default Value for a Column

To accomplish the same through pgAdmin, do this:

Step 1) Login to your pgAdmin account.

Step 2)

  1. From the navigation bar on the left- Click Databases.
  2. Click Demo.

Setting a Default Value for a Column

Step 3) Type the query in the query editor:

ALTER TABLE Book ALTER COLUMN book_author SET DEFAULT 'Nicholas Samuel';

Step 4) Click the Execute button.

Setting a Default Value for a Column

Step 5) To test, run the following command on the query editor:

INSERT INTO Book (id, name)
 VALUES (6, 'PostgreSQL for Beginners')

Step 6) Now, we can query the table to check whether the default value was inserted in the book_author column:

Setting a Default Value for a Column

Adding a Check Constraint

To accomplish the same through pgAdmin, do this:

Step 1) Login to your pgAdmin account.

Step 2)

  1. From the navigation bar on the left- Click Databases.
  2. Click Demo.

Adding a Check Constraint Using pgAdmin

Step 3) Type the query in the query editor:

ALTER TABLE Book ADD CHECK (book_author IN ('Nicholas', 'Samuel'))

Step 4) Click the Execute button.

Adding a Check Constraint Using pgAdmin

Step 5) To test this, do the following:

  1. Type the following query on the query editor: 
    INSERT INTO Book
VALUES(7, 'Best PostgreSQL Book', 'Gregory Bush');
  2. Click the Execute button.

    Adding a Check Constraint Using pgAdmin

    It will return the following:

Adding a Check Constraint Using pgAdmin

Renaming a Table

To accomplish the same through pgAdmin, do this:

Step 1) Login to your pgAdmin account.

Step 2)

  1. From the navigation bar on the left- Click Databases.
  2. Click Demo.

Renaming a Table Using pgAdmin

Step 3) Type the query in the query editor:

ALTER TABLE Book
  RENAME TO Books;

Step 4) Click the Execute button.

Renaming a Table Using pgAdmin

Step 5) To check whether the table was renamed, do the following:

  1. Click Databases from the left navigation.
  2. Expand Demo.
  3. Expand Schemas.
  4. Expand Public.
  5. Expand Tables.

Renaming a Table Using pgAdmin

The table was renamed successfully.

Summary

  • The ALTER TABLE statement is used to modify the structure of the table.
  • The ALTER TABLE command takes various forms depending on the task that you need to perform.
  • The structure can be the table columns or the table itself.
  • We can use this statement to change the name of a table.
  • The ALTER TABLE command can be used to set the default value of a column.
  • The statement can be used to validate the values that are entered into a table column.

Download the Database used in this Tutorial

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