Why Setting MySQL Columns to NOT NULL Boosts Performance and Saves Space

Query Optimization

When a column is defined as NOT NULL, MySQL knows that the column will never contain NULL and can skip NULL‑related checks during query execution, which speeds up reads, especially on large tables.

Reduce Storage Space

In InnoDB the default row format is COMPACT. This format stores a list of NULL values; if every column in a row is NOT NULL, the NULL‑value list disappears, saving one or more bytes per row. The following diagram shows the COMPACT layout (the dashed part is optional):

Index Efficiency

Columns that can be NULL make index optimization harder because NULL is a special value in MySQL’s B+‑tree: it is neither equal to nor greater/less than any other value, requiring extra logic in the optimizer. Queries that involve NULL may still use the index, but the processing is more complex and can be slower. col = 5 – normal index usage. col IS NULL – index can be used, but optimizer logic is more involved. col != 5 (or col <> 5) – if the column contains NULL, the result may be unpredictable and the optimizer might abandon the index.

In high‑concurrency scenarios, NOT NULL + a sensible default value is more stable than allowing NULL.

Avoid Aggregation Distortion

When a column contains NULL, aggregate functions behave differently: AVG, MAX, MIN, and SUM ignore NULL values, while COUNT treats them specially. For example, counting coupons that are not empty:

SELECT COUNT(*)
FROM orders
WHERE coupon_no != '';

If coupon_no can be NULL, the query must filter them explicitly:

SELECT COUNT(*)
FROM orders
WHERE coupon_no IS NOT NULL
  AND coupon_no != '';

Similarly, averaging scores that contain NULL will automatically skip those rows, inflating the average. To treat missing scores as zero, use IFNULL(score, 0).

Simplify Application Code

Defining a column as NOT NULL removes the need for repetitive null checks in code. In Java, a nullable age field requires:

if (user.getAge() == null) {
    // handle null
} else if (user.getAge() == 0) {
    // handle zero
}

By enforcing NOT NULL and using a default value (e.g., 0 for unknown age), the code becomes simpler and less error‑prone.

Improve Data Consistency

NOT NULL

constraints enforce that every row must contain a value for the column, preventing incomplete or inconsistent data. For example, a username column set to NOT NULL guarantees that every user record has a valid name.

Not All Scenarios Suit NOT NULL

Some fields naturally allow NULL to represent the absence of a value, such as a middle name, a deletion timestamp ( deleted_at), or optional remarks. In these cases, NULL is appropriate.

Conclusion

Overall, using NOT NULL for MySQL columns offers performance gains, storage savings, clearer code, and stronger data integrity, making it the preferred default unless the business logic explicitly requires nullable fields.