Add documentation on 'if not Insert then Modify' best practices

content/docs/BestPractices/if-not-insert-then-modify/index.md

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+---
+title: "if not Insert then Modify"
+tags: ["AL","Performance"]
+categories: ["Best Practice"]
+---
+
+_Created by SosicStefan, Described by SosicStefan_
+
+## Introduction
+
+When handling data insertion in Business Central, a common but inefficient pattern is attempting to insert a record and modifying it if the insert fails due to duplicate keys. This approach introduces unnecessary performance overhead and should be avoided.
+
+{{% alert title="Note" color="info" %}}
+This pattern is only good when used on singleton table. Read more about [Singleton Table](https://alguidelines.dev/docs/navpatterns/patterns/singleton/singleton-table/).
+{{% /alert %}}
+
+## Why Not Use 'If Not Insert Then Modify'?
+
+Using the following structure:
+
+```al
+InitializeRecord(SomeRecord);
+if not SomeRecord.Insert() then
+    SomeRecord.Modify();
+```
+
+Presents several issues:
+
+- Prevents buffered inserts: This impacts performance as batch insert optimizations are lost.
+- Generates unnecessary SQL errors: If a duplicate primary key exists, the failed insert raises an error, which Business Central must handle.
+- Slower and less efficient: Handling SQL errors is far from an optimal way to check for record existence.
+
+
+## Best Practice: Use IsEmpty
+
+Instead of relying on insert failures, ensure the record’s uniqueness upfront using a number series or an incremental primary key. If the data comes from an external source where duplicates might exist, use IsEmpty to check for existence.
+
+```AL
+InitializeRecord(SomeRecord);
+SomeRecord.SetRecFilter();
+
+if SomeRecord.IsEmpty() then
+    SomeRecord.Insert()
+else
+    SomeRecord.Modify();
+```
+
+## Why IsEmpty is Better
+
+- No SQL errors: The check is done before attempting an insert.
+- Performance improvement: Avoids unnecessary error handling and maintains buffered inserts.
+- More readable and efficient: The intent is clear—checking existence before performing actions.
+
+## Summary
+Avoid using if not Insert then Modify. Instead, ensure record uniqueness upfront or use IsEmpty to check existence efficiently. This leads to better performance and cleaner code.