So it is better to avoid the wildcard characters at the beginning of the pattern and only use them at the end if possible.
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In that case, a full table scan is required to match the pattern which consumes more database resources. However, they should not be used at the beginning of the pattern as this disables the database from using the index. Wildcard characters such as % and _ are used to filter out the results of a LIKE clause. Avoid Wildcard Characters at the beginning of a LIKE clause pattern ON Customers.CustomerID = Orders.CustomerID ġ0. It is better to use the Inner join in this case to obtain the same result. WHERE Customers.CustomerID = Orders.CustomerID So it is much better to use INNER JOIN as that only combines the rows from both tables which satisfy the required condition.Įxample: This is a query that displays the CustomerID of the customers that have currently ordered products using the WHERE clause. This is obviously problematic for large databases as more database resources are required. Using the WHERE clause for creating joins results in a Cartesian Product where the number of rows is the product of the number of rows of the two tables. Use INNER JOIN instead of WHERE clause for creating joins
It is better to use the WHERE clause in this case as it applies the condition to individual rows rather than the HAVING clause that applies the condition to the result from the GROUP BY clause.ĩ. Select distinct Customers.CustomerID, Customers.LastName from Customers INNER join Orders on Customers.CustomerID = Orders.CustomerID where Customers.Age = 25 AND age 25
This enhances the readability of the query and also makes reviewing and troubleshooting it easier. It is very important to provide the correct formatting while writing a query. Now that we have analyzed the tables Customers, Products and Orders, the different ways to optimize a query are given below with query examples from these tables: 1. Orders : Table Orders contains the details of the products ordered by the customers from the shop.ĬustomerID ProductID ProductQuantity 73001 1003 5 73001 1001 1 73003 1002 1 73004 1003 2 73004 1005 1 ProductID ProductName ProductPrice 1001 Shampoo 100 1002 Tooth paSHand SanitiDeodorant 100 Products : Table Products contains the details of the products available in the shop. CustomerID LastName FirstName Address Age 73001 Smith John 45 Jump Street 21 73002 Parker Anna 83 Wild Avenue 45 73003 James Josie 99 Chestnut Avenue 25 73004 White Anna 55 Paper Street 72 73005 Sparks Harry 11 Wisteria Lane 23 73006 Parker Jane 12 Quentin Road 50 Let us consider some sample tables to understand these different methods to optimize a query.Ĭustomers : Table Customers contains the details of the prospective customers for a shop. So it is very important to optimize the query to obtain the best database performance.
An inefficient query can drain the database resources, reduce the database speed or result in a loss of service for other users. However, there may be multiple SQL queries that yield the same results but with different levels of efficiency. ISRO CS Syllabus for Scientist/Engineer ExamĪn SQL Query is used to retrieve the required data from the database.ISRO CS Original Papers and Official Keys.GATE CS Original Papers and Official Keys.