How to Prevent Duplicate Values During Data Insertion in a \'Delegates\' Table?

Preventing Duplicate Values During Data Insertion

Problem Statement:

Within a table named "Delegates," which contains unique member information as well as range fields "FromYr" and "ToYr," data insertion using user inputs risks the duplication of members for the same year.

Solution:

To address this issue and ensure data integrity, we can leverage the MERGE statement. The MERGE statement allows us to perform a conditional insertion, updating the record only if it does not already exist within the table.

Implementation:

<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>

Explanation:

• MERGE INTO: This statement specifies the target table ("Delegates") to be updated or inserted into.
• USING: This clause defines the source data ("X") being compared against the target table.
• ON: The join criteria specifies the conditions under which rows should be matched during the merge operation. In this case, the unique key join (e.g., MemberNo and year) is used to identify when a row already exists.
• WHEN NOT MATCHED BY TARGET THEN: This statement defines the action to be taken if no matching row is found in the target table. In this case, an INSERT is performed to add the new row.

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