How to Prevent Duplicate INSERTs in SQL Using the MERGE Statement?

Preventing Duplicate INSERTs in SQL

Inserting data into a database table is crucial, but sometimes avoiding duplicate entries becomes a challenge. One such instance arises when inserting values provided by user input. In this scenario, users may mistakenly submit duplicate records, leading to data redundancy and integrity issues.

To address this problem, one effective solution is to utilize SQL's MERGE statement. The MERGE statement combines the functionality of INSERT, UPDATE, and DELETE operations into a single statement, providing fine-grained control over data manipulation. It allows you to insert new rows only if they do not already exist in the table.

For your specific table named "Delegates," with fields ID, MemberNo, FromYr, and ToYr, you can use the following MERGE statement to prevent duplicate insertions:

<code class="sql">MERGE INTO Delegates D
USING (values(@MemNo, @FromYr,@ToYr)) X ([MemNo],[FromYr],[ToYr])
ON (insert unique key join)
WHEN NOT MATCHED BY TARGET THEN
INSERT ([MemNo],[FromYr],[ToYr]))
VALUES (X.[MemNo],X.[FromYr],X.[ToYr]);</code>

In this query:

• The MERGE statement is used to insert new rows into the Delegates table.
• The USING clause specifies a virtual table (X) that contains the values to be inserted.
• The ON clause is empty in this case because we are not matching based on any existing rows.
• The WHEN NOT MATCHED BY TARGET clause specifies that the rows from the virtual table should be inserted if they are not already present in the Delegates table.

By using this MERGE statement, you can ensure that duplicate rows are not inserted into your Delegates table, preserving data integrity and preventing redundant records.

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