Sql update from select

Robin Day Robin Day If you are editing the the link between tables SET Table. CharlesWood yeah. I have the same question in MySQL. It would be great if someone knows how to implement it to MySQL and share with everyone. How do I use an alias in set? Sebastian covers a technique for this in a recent blog post: sqlity. Show 3 more comments. This was around 10x quicker than the equivalent update Merge bugs: mssqltips.

Your point? Show 5 more comments. DDiamond 1, 2 2 gold badges 6 6 silver badges 17 17 bronze badges. Jamal Jamal 7, 1 1 gold badge 11 11 silver badges 2 2 bronze badges. By far the simplest! This will tend to work across almost all DBMS which means learn once, execute everywhere. If that is more important to you than performance you might prefer this answer, especially if your update is a one off to correct some data.

So better than Robin Day's answer for this purpose. Thanks for sharing! Peter Mortensen This assumes none of the columns are nullable though. You're right, I was typing the example by hand. I've added a third and fourth clause to the where statement to deal with that. Col1, T1. Col1, T2. Col2 is more concise.

Depends on if you want to replace nulls in the destination with nulls from the source. Frequently, I don't. But if you do, Martin's construction of the where clause is the best thing to use.

Show 1 more comment. Col2, T2. Stewart 3, 3 3 gold badges 25 25 silver badges 36 36 bronze badges. Martin Smith Martin Smith k 81 81 gold badges silver badges bronze badges. Just an arbitrary alias in this case. Adrian Macneil Adrian Macneil Shiva Patrick Frenette Patrick Frenette 6 6 silver badges 2 2 bronze badges. Ryan Ryan 2, 6 6 gold badges 33 33 silver badges 45 45 bronze badges. I feel this should be the accepted answer, because it keeps things simple and to the point.

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.

In particular, we should consider this problem if we will update a large number of rows. To overcome this issue, we can disable or remove the index before executing the update query. On the other hand, a warning sign is seen on the Sort operator, and it indicates something does not go well for this operator. When we hover the mouse over this operator, we can see the warning details. During the execution of the query, the query optimizer calculates a required memory consumption for the query based on the estimated row numbers and row size.

However, this consumption estimation can be wrong for a variety of reasons, and if the query requires more memory than the estimation, it uses the tempdb data. This mechanism is called a tempdb spill and causes performance loss. The reason for this: the memory always faster than the tempdb database because the tempdb database uses the disk resources. Now, if we go back to our position, the MERGE statement can be used as an alternative method for updating data in a table with those in another table.

In this method, the reference table can be thought of as a source table and the target table will be the table to be updated.

The following query can be an example of this usage method. We have typed the Persons table after the MERGE statement because it is our target table, which we want to update, and we gave Per alias to it in order to use the rest of the query. With the help of this syntax, the join condition is defined between the target and source table. Discover why our customers rate Chartio 1. Sign up for a day free trial.

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