Welcome to evolving your database! Sometimes you need to add constraints to tables that already exist and contain data. This is where ALTER TABLE becomes your best friend.

Think of it like adding security systems to a house that's already built and occupied.

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Have you ever needed to add rules to something that was already in use?

The ALTER TABLE statement lets you modify existing table structures without rebuilding them. The basic syntax for adding constraints is:

This keeps your data intact while adding new protection rules.

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What's the advantage of modifying existing tables versus recreating them?

Adding a NOT NULL constraint to existing tables requires caution. If any rows already contain NULL values in that column, the constraint will fail.

You must clean up the data first or provide default values before adding NOT NULL.

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Why would the database reject adding NOT NULL if existing rows contain NULL values?

Here's how to safely add a NOT NULL constraint. First, update any NULL values, then add the constraint:

This two-step approach ensures success.

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What would happen if you skipped the UPDATE step?

Adding UNIQUE constraints is similar. If duplicate values exist, the constraint addition will fail. You need to resolve duplicates first.

Check for duplicates, decide which to keep, then add your UNIQUE constraint.

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