MySQL primary key constraints and uniqueness constraints

MySQL primary key constraints and uniqueness constraints

MySQL primary key constraints and uniqueness constraints are both indexes. The differences between them are:

1. The primary key field can be unique, but the primary key field cannot be NULL.
2. The Uniqueness constraint ensures uniqueness, but the field of the uniqueness constraint can be NULL.
3. The Uniqueness constraint does not apply to records with NULL values. That is, records with NULL values can be added repeatedly. The primary key field cannot be NULL.

Mysql-workbench cannot create multi-field uniqueness constraints. You can manually add them on the command line.
Added multi-field uniqueness constraints
Mysql> alter table pai_end_regexp add constraint dev_series_uniq UNIQUE (dev_category_id, dev_series_id, dev_type_id );

Delete uniqueness Constraint
Mysql> alter table pai_end_regexp drop index dev_series_uniq;

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