Set a Default Value for an Existing SQL Column

When working with databases, it’s common to update an existing table and assign a default value to a column. Default values ensure that whenever a new row is inserted without specifying that field, the database automatically fills it in.

This improves data consistency, reduces errors, and makes database management easier.

In this guide, you’ll learn how to set or change a default value for an existing column in the most popular SQL databases—including MySQL, PostgreSQL, SQL Server, and SQLite.

How to Set Default Values in Different SQL Databases

SQL Server

ALTER TABLE table_name
ADD CONSTRAINT df_table_column DEFAULT 'new_default' FOR column_name;

MySQL

ALTER TABLE table_name
ALTER COLUMN column_name SET DEFAULT 'new_default';

PostgreSQL

ALTER TABLE table_name
ALTER COLUMN column_name SET DEFAULT 'new_default';

SQLite

SQLite does not allow modifying a column’s default directly. You must recreate the table:

1. Create new table with correct default
2. Copy data
3. Drop old table
4. Rename new table

Best Practices When Changing Default Values

1. Always back up your database before altering schema
2. Test changes in a staging environment
3. Ensure your application logic supports the new default
4. Avoid unnecessary defaults on large, high-write tables
5. Document schema changes for future maintainers

Summary

Setting a default value on an existing SQL column is a straightforward task in most database systems, but the exact syntax varies. MySQL, PostgreSQL, and SQL Server support it with simple ALTER TABLE commands, while SQLite requires recreating the table.

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