Ensuring Unique Row Inserts in SQL Databases Under Pressure
A common challenge for SQL developers is inserting rows without creating duplicates. The traditional method often involves a subquery:
<code class="language-sql">INSERT INTO TheTable SELECT @primaryKey, @value1, @value2 WHERE NOT EXISTS (SELECT NULL FROM TheTable WHERE PrimaryKey = @primaryKey)</code>
Under heavy load, however, this approach can lead to primary key violations, compromising statement atomicity.
To mitigate this, some developers employ locking mechanisms:
<code class="language-sql">INSERT INTO TheTable WITH (HOLDLOCK, UPDLOCK, ROWLOCK) SELECT @primaryKey, @value1, @value2 WHERE NOT EXISTS (SELECT NULL FROM TheTable WITH (HOLDLOCK, UPDLOCK, ROWLOCK) WHERE PrimaryKey = @primaryKey)</code>
But excessive locking significantly impacts performance.
A more efficient strategy, often referred to as the "JFDI" (Just F***ing Do It) method, involves direct insertion with error handling:
<code class="language-sql">BEGIN TRY INSERT etc END TRY BEGIN CATCH IF ERROR_NUMBER() <> 2627 RAISERROR etc END CATCH</code>
This avoids nested queries and locking, promoting concurrency under high volume.
Critically, proper indexing is paramount. As highlighted in "Lesson 4" by a seasoned SQL developer, relying solely on subqueries for duplicate prevention is unreliable. A unique index, coupled with appropriate error handling (like catching error 2627), provides automatic duplicate prevention, streamlining the process.
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