Delete duplicate records from a SQL table without a primary key

Add a unique identifier: A temporary superpower

While dealing with duplicates in a table with no primary key, consider bestowing temporary unique identity to each row. This mimics a primary key and aids in eradicating duplicates:

-- Become Iron Man and forge your table a new ARC Reactor (ID). 
-- Warning: You may not feel so good during the process.
ALTER TABLE your_table ADD id INT IDENTITY(1,1);


Post removing duplicates, this auxiliary column may be removed:

-- The ID has served its purpose, time for it to retire like Cap.
ALTER TABLE your_table DROP COLUMN id;


This ensures you don't accidentally delete valid data during your duplicate hunt.

Delete duplicates maintaining original structure

Sometimes we can't modify the table structure due to database restrictions or design constraints. Use a self-join on duplicate identifying columns:

-- Sending duplicates to the Upside Down world. Stranger Things have happened!
DELETE t1
FROM your_table t1
INNER JOIN your_table t2 
WHERE t1.duplicateColumn = t2.duplicateColumn
AND t1.uniqueColumn > t2.uniqueColumn;


Replace duplicateColumn with columns indicating duplicates and uniqueColumn with a unique column, like a timestamp.

DBMS specific strategies: Avengers, assemble!

Different databases, different strategies. Design compatibility is key.

SQL Server

Use ROW_NUMBER(), partitioning by duplicate criteria, ordering by a unique column (when available):

-- For SQL Server fans, think of this as your Ant-Man's Quantum Realm maneuver.
WITH cte AS (
  SELECT *, ROW_NUMBER() OVER (PARTITION BY DuplicateColumn ORDER BY UniqueColumn) AS rn
  FROM your_table
)
DELETE FROM cte WHERE rn > 1;


PostgreSQL & Oracle

For PostgreSQL and Oracle, use CTID or ROWID which are unique identifiers:

-- It's like sending duplicate rows to the Space Stone. They won't return!
DELETE FROM your_table
WHERE ctid NOT IN (
  SELECT min(ctid)
  FROM your_table
  GROUP BY DuplicateColumn
);


Put your duplicate indicating column in place of DuplicateColumn.

MySQL

In MySQL, use a temporary table. Keep unique, drop duplicates:

-- Think of it as the Time Stone strategy. You go back, preserve what's needed, and alter the future!
CREATE TEMPORARY TABLE temp_table AS
SELECT * FROM your_table
GROUP BY DuplicateColumn;

TRUNCATE your_table;

INSERT INTO your_table SELECT * FROM temp_table;


This makes sure only the first occurrence survives.

Data integrity: The OG Avengers

Retaining data consistency and integrity post duplicate deletion is vital. Always test the queries in a safe playing ground before running on the main field. Validate:

-- Regular checkups are necessary, not only for humans but also for databases.
SELECT columns, COUNT(*)
FROM your_table
GROUP BY columns
HAVING COUNT(*) > 1;


Running these before and after ensures you keep only the records you want!

Practice caution: The tesseract isn't a toy!

Supervising large datasets

Dealing with large tables is like handling the Hulk. Manage with batches for efficiency and server responsiveness.

Cross-table integrity

If removing duplicates affects interconnected tables, remember, Captain America won't approve destroying innocent data!

Regular duplicates

If your table was hit by a "Duplicate Bomb", there are some Thanos level villainous forces at work. Kill it at the source with constraints or checks on your inserts and updates. Add unique index on columns involved:

-- Screenshot this and let it define your Core values. Get it? "Core"? "Index"? I'll see myself out.
CREATE UNIQUE INDEX idx_uniq_col1_col2 ON your_table(col1, col2);


Secure your table's future against duplicate invasions!