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-- View all clients
SELECT * FROM clients;

-- Add a new column for client phone numbers
ALTER TABLE clients
ADD COLUMN phone TEXT;

-- View all clients
SELECT * FROM clients;

-- Update phone for one of the clients
UPDATE clients
SET phone = '+1-555-123-4567'
WHERE name = 'Jason Green';

-- View all clients
SELECT * FROM clients;

-- Rename the column from phone to contact_number (SQLite only)
ALTER TABLE clients
RENAME COLUMN phone TO contact_number;

-- View all clients
SELECT * FROM clients;

# Altering Tables

The `ALTER TABLE` command lets you **modify the structure** of an existing table without deleting or recreating it.

You can add, rename, or remove columns — all while keeping your existing data intact.

<br/>

## Add a Column

To add a new column to an existing table, use the `ADD COLUMN` command.

```sql title="Add a new phone column"
ALTER TABLE clients
ADD COLUMN phone TEXT;
```

This adds a `phone` column to the `clients` table to store contact numbers.

<br/>

## Rename a Column

Renaming a column helps make names more descriptive or consistent with your data.

Syntax can vary between database systems.

For example, SQLite and PostgreSQL support the `RENAME COLUMN` command, while MySQL uses `ALTER TABLE RENAME COLUMN`.

Below is an example of how to rename the `phone` column to `contact_number` in the `clients` table.

```sql title="Rename the phone column"
ALTER TABLE clients
RENAME COLUMN phone TO contact_number;
```

This command renames the `phone` column to `contact_number` in the `clients` table.

> *Note:* Some databases require different syntax or do not support this operation at all.

<br/>

## Change a Column's Data Type

Some SQL systems let you change a column’s data type using `ALTER COLUMN`:

```sql title="Change data type of phone"
ALTER TABLE clients
ALTER COLUMN contact_number TYPE TEXT;
```

> *Note*: SQLite, the database engine used at CodeFriends, doesn’t support `ALTER COLUMN TYPE` directly. You’d need to recreate the table or copy data into a new structure instead.

<br/>

## Drop a Column

To permanently remove a column, you can use the `DROP COLUMN` command.

```sql title="Remove the contact_number column"
ALTER TABLE clients
DROP COLUMN contact_number;
```

> *Be careful*: Dropping a column permanently deletes its data.

`ALTER TABLE` allows you to add, rename, modify, or drop columns in an existing table.