Therefore, the target table gets updated with the reference columns data for the specified conditions. The MERGE statement is useful for manipulating data in the target table based on the source table data for both matched and unmatched rows. It is a straightforward method to update the existing table data from other tables.
The merge join uses an inner join for matching data rows between the source and target data. It also has the maximum relative cost for sort operator. The subquery is the fastest method to update column data. It uses the clustered index update and clustered index scan as highlighted. The subquery works efficiently, but it has its own limitations, as highlighted earlier. The overall performance of your database depends on the table data, the number of updates, table relationships, indexes , and statistics.
SQL Server execution plans are a gateway for query optimizer and query executions. Learn how to read, interpret and analyze execution plan data. Here are three key areas to focus your SQL Server performance tuning efforts to keep your database in peak condition. Mostly, we use constant values to change the data, such as the following structures. The full update statement is used to change the whole table data with the same value. This type of update statement is a bit complicated than the usual structures.
In the following sections, we will learn how to write this type of update query with different methods, but at first, we have to prepare our sample data. With the help of the following query, we will create Persons and AddressList tables and populate them with some synthetic data. These two tables have a relationship through the PersonId column, meaning that, in these two tables, the PersonId column value represents the same person.
In this method, the table to be updated will be joined with the reference secondary table that contains new row values. So that, we can access the matched data of the reference table based on the specified join type. Lastly, the columns to be updated can be matched with referenced columns and the update process changes these column values.
After the execution of the update from a select query the output of the Persons table will be as shown below;. After the SET keyword, we specified the column names to be updated, and also, we matched them with the referenced table columns.
After the FROM clause, we retyped the table name, which will be updated. In addition to this, we can specify a WHERE clause and filter any columns of the referenced or updated table. We can also rewrite the query by using aliases for tables. Indexes are very helpful database objects to improve query performance in SQL Server. Particularly, if we are working on the performance of the update query, we should take into account of this probability. The following execution plan illustrates an execution plan of the previous query.
The only difference is that this query updated the 3. This query was completed within 68 seconds. We added a non-clustered index on Persons table before to update and the added index involves the PersonCityName and PersonPostCode columns as the index key.
The following execution plan is demonstrating an execution plan of the same query, but this query was completed within seconds because of the added index, unlike the first one. We have seen this obvious performance difference between the same query because of index usage on the updated columns. As a result, if the updated columns are being used by the indexes, like this, for example, the query performance might be affected negatively.
The end result of our MERGE statement is that for every author in the authors table, we verify whether a corresponding book exists in books. If a record is found, we ensure books. There are many database diagramming tools that are not only incredibly useful but also free. After launching and connecting to SQL Server Management Studio, create a new login and select the database that is connected to Chartio.
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