SQL Triggers: Trigger Select into After Update/Insert

In this article, we will explore the concept of SQL triggers and how to use them to perform a SELECT statement after an update or insert operation on a table. We will focus on creating a trigger that inserts selected data from the updated Audit_Data table into the Audit_Final table.

Understanding SQL Triggers

A SQL trigger is a stored procedure that is automatically executed by the database management system (DBMS) in response to certain events, such as an update or insert operation. Triggers can be used to enforce business rules, perform calculations, or insert data into other tables.

In this article, we will create a trigger that performs a SELECT statement on the updated Audit_Data table and inserts selected data into the Audit_Final table.

Requirements

Before creating the trigger, let’s review the requirements:

• The Audit_Data table has 300 rows with around 20 columns per row.
• The updated rows share the same Audit_ID.
• We need to create a trigger that performs a SELECT statement on the updated Audit_Data table and inserts selected data into the Audit_Final table.

Creating the Trigger

To create the trigger, we will use the following SQL script:

CREATE TRIGGER [TR_Audit_Data] ON [Audit_Data]
AFTER UPDATE
AS
BEGIN
    INSERT INTO [Audit_Final] (
        /* column1, */
        /* column2, */
        /* ... all columns you have on target table */
    )
    SELECT 
        /* Select columns from main table */
        /* main.Audit_ID, */
        /* main.Item_19, */
        /* ... select other relevant columns from main table */

        /* Select columns from macaddr table */
        /* macaddr.Item_2, */
        /* macaddr.Item_16, */
        /* ... select other relevant columns from macaddr table */

        /* Select columns from t1 table */
        /* t1.Item_1, */
        /* ... select other relevant columns from t1 table */
    FROM 
        dbo.[Audit_Data] AS main
        LEFT JOIN (
            SELECT Audit_ID, Item_2, Item_16
            FROM dbo.[Audit_Data] AS macaddr
            WHERE (Item_2 NOT LIKE 'Hyper-V%')
                AND (Item_17 = 'connected') 
                AND (Item_18 IN ('10000Mbps', '1000MBps') OR ITEM_9 IS NOT NULL AND ITEM_10 IS NOT NULL)
                AND (Item_18 != '100Mbps')
        ) macaddr ON main.Audit_ID = macaddr.Audit_ID
        LEFT JOIN (
            SELECT Audit_ID, Category_ID, Item_1, Record_ordinal
            FROM dbo.[Audit_Data] AS t1
            WHERE 
                Item_1 = 'Automatyczna konfiguracja sieci przewodowej' OR Item_1 = 'dot3svc' OR Item_1 = 'Wired AutoConfig'
                AND Item_3 = 'Running'
                AND Category_ID = '4100'
        ) t1 ON main.Audit_ID = t1.Audit_ID
    WHERE 
        main.Record_Ordinal = '2'
    ORDER BY main.Audit_ID;
END;

Note that we need to replace the /* column1, */ comments with the actual column names in the Audit_Final table.

Understanding the Trigger Script

The trigger script consists of two parts:

1. The INSERT INTO statement: This statement inserts selected data from the updated Audit_Data table into the Audit_Final table.
2. The SELECT statement: This statement selects relevant columns from the updated Audit_Data table and joins them with other tables (macaddr and t1) based on the Audit_ID.

The SELECT statement uses a LEFT JOIN to combine data from multiple tables, ensuring that all rows are included in the result set.

Example Use Case

Suppose we update the Record_Ordinal column of two rows in the Audit_Data table:

UPDATE dbo.[Audit_Data]
SET Record_Ordinal = '2'
WHERE Audit_ID = 1 AND Item_19 = 'value1';

The trigger script will execute automatically and insert selected data from the updated Audit_Data table into the Audit_Final table.

Conclusion

In this article, we explored how to create a SQL trigger that performs a SELECT statement after an update or insert operation on a table. We created a trigger that inserts selected data from the updated Audit_Data table into the Audit_Final table and demonstrated how to use the trigger script. This technique can be used to enforce business rules, perform calculations, or insert data into other tables based on database events.

Additional Tips and Variations

• To avoid inserting duplicate rows, you can add a unique constraint on the columns being inserted into the Audit_Final table.
• To improve performance, consider using an index on the columns used in the trigger script.
• To make the trigger more flexible, consider using parameters to pass in the values being updated or inserted.
• To use this technique with other database systems, such as PostgreSQL or MySQL, you may need to modify the trigger script accordingly.

Last modified on 2023-05-22