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SQL ALTER TABLE Statement
SQL ALTER TABLE Statement
Learn how to use ALTER TABLE to modify existing database tables and maintain data integrity.
What Is the ALTER TABLE Statement?
The ALTER TABLE command allows you to modify the structure of an existing table. This includes adding new columns, modifying existing ones, or adding constraints to maintain data integrity.
Basic Syntax
Real-World Examples with TokTuk Tables
Adding New Columns
Let's add some useful columns to our existing tables:
Adding Constraints
We can add constraints to ensure data integrity:
Modifying Existing Columns
Sometimes we need to modify existing columns:
Common ALTER TABLE Operations
Adding Columns
Use ADD COLUMN to extend your table with new fields. Always consider whether to make the column nullable or provide a default value to handle existing rows.
Adding Constraints
Add constraints like CHECK, UNIQUE, or FOREIGN KEY to enforce data rules. Make sure existing data complies with new constraints before adding them.
Modifying Columns
Change column properties like data type or constraints. Be careful with type changes that might cause data loss or conversion issues.
Best Practices for ALTER TABLE
- Always backup your data before making schema changes
- Test changes in a development environment first
- Consider the impact on existing queries and applications
- Plan for data migration when modifying columns
- Add appropriate indexes for new columns used in WHERE clauses
- Document all schema changes for future reference
- Consider database locking and downtime requirements
- Verify data integrity after changes are complete
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