Using the Merge Statement to populate a historical table with Effective Dating

One of my favorite uses for the MERGE statement introduced in SQL Server 2008 is the updating of a historical table. With versions prior to 2008 this operation had to be performed in two separate statements. Merge helps us to streamline the process. The advantage to the database engine when using a Merge statement is in it’s minimal locking. Prior to merge, we had to update the record if it existed. And if it did not exist, we inserted the record. Long story short, it was double the amount of locking before the optimizer knew which rows to exclusively lock. To understand what’s going on in the statement below, you have to read starting from the MERGE statement. This is where the initial updating is taking place first. As you scroll down, you will see the OUTPUT clause. This actually outputs the results of the MERGE statment. You can think of this output as a virtual table named “merged”. The $action variable is an intrinsic column (if you will), that contains the action that row played during the MERGE. Lastly, if you scroll to the top, you will find the insert statement that finally inserts the new “current” record for the corresponding “expired” record we just retired. This was the same row we used for the criteria to UPDATE the expired row, however we did not use any of it’s values. [cc lang=”sql”] DECLARE @Now datetime = GETDATE() DECLARE @EffToDate datetime = ‘2079-06-06T00:00:00.000’ DECLARE @JobID int = 1 — This […]

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