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SQL & Databases Masterclass Course

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LESSON WORKSPACEAdvanced

SQL Lesson 17: Altering tables

Introduction & Core Concept

As software applications mature, business requirements evolve. You might need to add a new phone number column to customer profiles or attach tracking tags to product ledgers.

To modify the structural schema of an active, populated table without dropping and recreating it from scratch, we use the ALTER TABLE statement.

ALTER TABLE table_name 
ADD COLUMN column_name DATATYPE;
Why & Where We Use It
  • Why We Use It: Preserves active operational data while expanding database capabilities. You can seamlessly attach new columns to tables containing millions of rows in real time.
  • Where We Use It: Database schema migrations, expanding user profile attributes, and supporting new software feature releases.
    • Real-World Example

    ShopMart decides to launch an email newsletter marketing campaign. Their existing users table only tracks names and locations. The database administrator runs an ALTER TABLE query to append an email_address text column to the active table.

    Best Practices: What to Do & What NOT to Do
  • What to Do: When adding new columns to existing populated tables, assign a DEFAULT value clause. This automatically populates historical rows with a clean baseline value rather than leaving millions of cells filled with NULL.
  • What NOT to Do: Avoid altering tables during peak traffic hours. Altering massive tables can place exclusive schema locks on the database, temporarily blocking incoming read and write transactions!
    • Syntax & Pro Tips
    ALTER TABLE users ADD COLUMN email_address TEXT DEFAULT 'no-email@company.com';
    Interactive Sandboxed Terminal (Preloaded DB Schema: SHOPMART)
    SQL Query WorkspaceSQLite v3.45 (WASM Mode)
    QUICK INSERT:
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