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Replacing cursors using joins in sql server - Part 64

Suggested Videos
Part 61 - Creating a large table with random data for performance testing
Part 62 - What to choose for performance - SubQuery or Joins
Part 63 - Cursors in sql server

In Part 63, we have discussed about cursors. The example, in Part 63, took around 45 seconds on my machine. Please watch Part 63, before proceeding with this video. In this video we will re-write the example, using a join.

Update tblProductSales
set UnitPrice = 
When Name = 'Product - 55' Then 155
When Name = 'Product - 65' Then 165
When Name like 'Product - 100%' Then 10001
from tblProductSales
join tblProducts
on tblProducts.Id = tblProductSales.ProductId
Where Name = 'Product - 55' or Name = 'Product - 65' or 
Name like 'Product - 100%'

When I executed this query, on my machine it took less than a second. Where as the same thing using a cursor took 45 seconds. Just imagine the amount of impact cursors have on performance. Cursors should be used as your last option. Most of the time cursors can be very easily replaced using joins.

To check the result of the UPDATE statement, use the following query.
Select  Name, UnitPrice from 
tblProducts join
tblProductSales on tblProducts.Id = tblProductSales.ProductId
where (Name='Product - 55' or Name='Product - 65' or 
Name like 'Product - 100%')

1 comment:

  1. when I type out the query myself the query to check the data after update and I paste yours, I get 2 different returned sets and the queries are the same


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