The Windows binary distribution works out of the box with SQLite, MSSQL, PostgreSQL and MySQL. The Gotchas section at the end of this chapter has some more information about specific databases. Google NoSQL is treated as a particular case in Chapter 13. Please check on the web2py web site and mailing list for more recent adapters. A partial list of supported databases is show in the table below. The DAL dynamically generates the SQL in real time using the specified dialect for the database back end, so that you do not have to write SQL code or learn different SQL dialects (the term SQL is used generically), and the application will be portable among different types of databases. I am always interested in new challenges so if you need consulting help, reach me at all posts by Rajendra GuptaWeb2py comes with a Database Abstraction Layer (DAL), an API that maps Python objects into database objects such as queries, tables, and records. I am the creator of one of the biggest free online collections of articles on a single topic, with his 50-part series on SQL Server Always On Availability Groups.īased on my contribution to the SQL Server community, I have been recognized as the prestigious Best Author of the Year continuously in 2019, 2020, and 2021 (2nd Rank) at SQLShack and the MSSQLTIPS champions award in 2020. I published more than 650 technical articles on MSSQLTips, SQLShack, Quest, CodingSight, and SeveralNines. I am the author of the book " DP-300 Administering Relational Database on Microsoft Azure". Hi! I am Rajendra Gupta, Database Specialist and Architect, helping organizations implement Microsoft SQL Server, Azure, Couchbase, AWS solutions fast and efficiently, fix related issues, and Performance Tuning with over 14 years of experience. You can replace SQL COUNT DISTINCT with the keyword Approx_Count_distinct to use this function from SQL Server 2019. There might be a slight difference in the SQL Count distinct and Approx_Count_distinct function output. This new function of SQL Server 2019 provides an approximate distinct count of the rows. SQL Server 2019 improves the performance of SQL COUNT DISTINCT operator using a new Approx_count_distinct function. In a table with million records, SQL Count Distinct might cause performance issues because a distinct count operator is a costly operator in the actual execution plan. In the properties windows, also we get more details around the sort operator including memory allocation, statistics, and the number of rows. You can hover the mouse over the sort operator, and it opens a tool-tip with the operator details. In this execution plan, you can see top resource consuming operators: You need to enable the Actual Execution Plan from the SSMS Menu bar as shown below.Įxecute the query to get an Actual execution plan. Let’s look at the Actual Execution Plan of the SQL COUNT DISTINCT function. It does not eliminate the NULL values in the output.Įxecution Plan of SQL Count distinct function It eliminates the NULL values in the output. It gives a distinct number of rows after eliminating NULL and duplicate values. It considers all rows regardless of any duplicate, NULL values. It returns the distinct number of rows after satisfying conditions specified in the where clause. It returns the total number of rows after satisfying conditions specified in the where clause. Similarly, you can see row count 6 with SQL COUNT DISTINCT function.ĭifference between SELECT COUNT, COUNT(*) and SQL COUNT distinct In the output, we can see it does not eliminate the combination of City and State with the blank or NULL values. Let’s create a sample table and insert few records in it. It also includes the rows having duplicate values as well. It means that SQL Server counts all records in a table. We can use SQL Count Function to return the number of rows in the specified condition.īy default, SQL Server Count Function uses All keyword. We want to know the count of products sold during the last quarter. Suppose we have a product table that holds records for all products sold by a company. We use SQL Count aggregate function to get the number of rows in the output. Let’s go ahead and have a quick overview of SQL Count Function. Suppose we want to get distinct customer records that have placed an order last year. Sometimes, we want to get all rows in a table but eliminate the available NULL values. This article explores SQL Count Distinct operator for eliminating the duplicate rows in the result set.Ī developer needs to get data from a SQL table with multiple conditions.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |