DEFAULT

The DEFAULT constraint assigns a predefined value to a column when you insert a row without providing a value for that column.

It reduces repetitive INSERT logic and ensures consistent data across records.

What a DEFAULT Constraint Does

DEFAULT supplies a value automatically during INSERT.

The Status becomes 'Pending' and CreatedAt becomes today's date.

Defining a DEFAULT Constraint

Set it during table creation:

Add it later:

Remove it later:

DEFAULT Values

DEFAULT supports:

• Constants: 'Active', 0, 'N/A'
• Expressions: CURRENT_DATE, CURRENT_TIMESTAMP, NOW()

Real-World Uses

Pseudocode example:

Frequently Asked Questions

Q: Does DEFAULT apply to UPDATE statements? A: No. DEFAULT applies only when a column is omitted in an INSERT. Use COALESCE in an UPDATE if needed.

Q: Can DEFAULT return different values per row? A: Yes, when using expressions like CURRENT_DATE or NOW(), the database evaluates the expression per row.

Q: What happens if I explicitly insert NULL? A: DEFAULT is ignored. The column stores NULL, which will fail if it is also NOT NULL.

Summary

• DEFAULT provides automatic values when none are supplied.
• Use it to reduce boilerplate and enforce consistent initial states.
• Combine it with NOT NULL to guarantee a usable value.
• Common defaults include status flags and timestamps.

Next Step

See Composite Keys to combine multiple columns into a primary key, or return to the SQL Constraints overview.