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Duplicate key on write or update?

sql
data-integrity
database-design
constraint-naming
Nikita BarsukovbyNikita Barsukov·Dec 21, 2024
TLDR

To sidestep the Duplicate key error, stick to conditional SQL operations. For inserts, become comfortable with the ON DUPLICATE KEY UPDATE clause:

-- Regular inserts go BOOM! when they see duplicates. -- But not our hero, "ON DUPLICATE KEY UPDATE" INSERT INTO table_name (unique_column, other_column) VALUES ('unique_value', 'value') ON DUPLICATE KEY UPDATE other_column = VALUES(other_column);

And for updates, evade clones by embedding a subquery that validates the new value doesn't already exist in the unique column:

-- Updates with their magnifying glasses out for existing unique values UPDATE table_name SET unique_column = 'new_unique_value' WHERE id = some_id AND 'new_unique_value' NOT IN (SELECT unique_column FROM table_name WHERE id != some_id);

Insertion operations embrace duplicates with INSERT ... ON DUPLICATE KEY UPDATE, whereas updates demand an ancillary non-existence verification to swerve collisions.

Deciphering error 121, aka "Duplicates' Trojan horse"

Stumbling upon error 121 in MySQL? You're playing against duplicate CONSTRAINT names. A constraint is a part of SQL Royal Family, they too demand uniqueness within the kingdom (a.k.a your database).

-- Constraint naming 101: As unique as a snowflake in an avalanche ALTER TABLE your_table_name ADD CONSTRAINT constraint_name_unique FOREIGN KEY (column_name) REFERENCES another_table(id);

To wrestle down error 121, authenticate that constraint names are as unique as your coffee order. Renaming your constraint can settle this muddle:

-- Apparently, constraints are as egoistic as popstars. One name. Their own. No duplicates. Period. ALTER TABLE your_table_name DROP FOREIGN KEY old_constraint_name, ADD CONSTRAINT new_constraint_name_unique FOREIGN KEY (column_name) REFERENCES another_table(id);

Times when key integrity takes precedence

Data Integrity comes with a combative spirit for key constraints triggering errors during inserts or updates:

  • Primary Key Demolition: An attempt to sneaking in a doppelganger value into primary key pit.
  • Foreign Key Inconsistency: Playing seek-&-hide with a value in a foreign key field, not found in the parent table.
  • Unique Key Duplicates: Redesigning a non-primary key attribute with a value that's photocopying another record.

Tuning design to prevent duplicate keys

Precautions: Craft a fault-tolerant database schema with unique constraints:

  1. Primary Keys: Auto-incremented integers or UUIDs for a guaranteed uniqueness.
  2. Unique Indexes: Tailored for global uniqueness such as email addresses.
  3. Foreign Key Constraints: Ensuring data harmony with well-articulated constraints.

Probing Inserts: Like a security check at the airport, perform dry runs to sniff out potential violations:

-- We are like airport security for databases, checking for potential threats SELECT COUNT(*) FROM table_name WHERE column_name = 'potential_unique_value';

Any non-zero count presents a signal of duplicate key error.

Schema Review: Schedule your database for a constraint names audit. A tad bit of tidying up post-cloning or refactoring tables might help avoid the error 121.

Strategies to cope with the key conflict mayhem

Exception Handling: Let your application handle SQL exceptions related to key violations with grace and informative feedback.

Keeping Retries Handy: Modify your logic to append a uniquifier (timestamp/random string) and retry the operation in case of a conflict.

Normalization: Keep the data redundancy at bay with database normalization to drive down key violation risks.