How to fix MariaDB 4131 NULL column used in unique index (check configuration)?

Add NOT NULL constraint to the indexed column: Making the column NOT NULL ensures unique index behavior is well-defined.

4131

MariaDBERRORNotableMEDIUM confidence

NULL column used in unique index (check configuration)

Production Risk

Low — DDL fails; no data is affected.

Why it happens

1A column used in a unique index is NULL-able and the storage engine does not support it in this configuration.
2The index context requires NOT NULL columns.

Fix

Add NOT NULL constraint to the indexed column

ALTER TABLE t MODIFY col INT NOT NULL;

Why this works

Making the column NOT NULL ensures unique index behavior is well-defined.

What not to do

Sources

MySQL 8.0 — 4131 ER_NULL_COLUMN_IN_INDEX2

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At a glance

PlatformMariaDB Error Codes

Code4131

SeverityERROR

TierNotable

ConfidenceMEDIUM