How to Avoid 'Incorrect syntax near the keyword 'WHERE'' When Inserting Unique Data in SQL Server?

SQL Server: Preventing Duplicate Entries with Conditional Inserts

When working with SQL Server databases, preventing duplicate data insertion is crucial for maintaining data integrity. A common approach involves using conditional inserts, often encountering syntax issues. The error "Msg 156, Level 15, Incorrect syntax near the keyword 'WHERE'" frequently arises from incorrect implementation of "Insert if not exists" logic.

The solution lies in restructuring the insert statement. Instead of a direct "Insert if not exists" attempt, use a conditional block:

<code class="language-sql">BEGIN
   IF NOT EXISTS (SELECT 1 FROM EmailsRecebidos 
                   WHERE De = @_DE
                   AND Assunto = @_ASSUNTO
                   AND Data = @_DATA)
   BEGIN
       INSERT INTO EmailsRecebidos (De, Assunto, Data)
       VALUES (@_DE, @_ASSUNTO, @_DATA)
   END
END</code>

This revised code uses IF NOT EXISTS to check for the existence of a record matching the specified criteria (De, Assunto, Data) before attempting an insertion. This prevents duplicate entries. Note the use of SELECT 1 for efficiency; it only needs to find one matching row, not retrieve all columns.

While this method effectively handles most scenarios, it's important to acknowledge the potential for race conditions in high-concurrency environments. To ensure absolute data integrity, consider more robust techniques such as stored procedures, triggers, or appropriate locking mechanisms to manage concurrent access and prevent data inconsistencies.

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