![]() Note: I've cleaned up the psql output in these examples to make it easier to read, so don't worry if the output shown here isn't exactly what you've seen in your terminal. You are now connected to database "fcc" as user "john". Next let's start the interactive console by using the command psql, and connect to the database we just made using \c : $ psql With PostgreSQL already installed, we can run the command createdb at our terminal to create a new database. If you have another database client that you enjoy working with that's fine too. To work with our PostgreSQL database, we can use psql-the interactive PostgreSQL command line program. Setting up example data (creating sales)īefore we can write our queries we need to setup our database.įor these examples we'll be using PostgreSQL, but the queries and concepts shown here will easily translate to any other modern database system (like MySQL, SQL Server, and so on).In this article we'll look at how to construct a GROUP BY clause, what it does to your query, and how you can use it to perform aggregations and collect insights about your data. The numbers will be different when you run it because we ARE using random constrained data here.The GROUP BY clause is a powerful but sometimes tricky statement to think about.Įven eight years later, every time I use a GROUP BY I have to stop and think about what it's actually doing. Here's what the first several rows of the results look like. ![]() ,TotalAmount = CASE WHEN TransactionDescription = 'Negative' THEN -Amount ELSE Amount END It will make your server more secure and easier to do things with. 2008 is now 14 years old and it hasn''t been supported for years. One solution using the technology in SQL Server 2008. ![]() While I appreciate the use of CROSS APPLY (and it IS supported in 2008), I think it's an unnecessary complication here. = See what the first 100 rows look like.Īnd here's an easy solution. ,Adjustment = CONVERT(DECIMAL(9,2),RAND(CHECKSUM(NEWID()))*10.0)įROM sys.all_columns ac1 -"Pseduo-Cursor" replaces a loop ,TransactionDescription = IIF(ABS(CHECKSUM(NEWID())%2) = 1, 'SomethingElse','Negative') = Create and populate the test table on-the-fly with a million rows of data. We''re just making a populated test table here. This has NOTHING to do with the solution. This takes less than a seond on my laptop. See the following links for how this works. Create a table with some random but constrained test data. I normally use my dbo.fnTally() function for this instead of the CROSS JOIN but one lesson at a time. I've included two articles in the header below for how it works. Because it's basically unlimited in the number of rows you can quickly create, it's great for performance testing when you don't have enough real data. You really need to read the article at the first link in my signature line below for how to provide some "readily consumable" data. (SELECT TransactionID, Amount, TransactionDescription, RunningSum = SUM(t.RepairedAmount) OVER (ORDER BY t.TransactionID ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) If you're trying to do a running total (seems like of what I'm calling "RepairedAmount"), then you need to use a windowing function, like this: SELECT t.TransactionID, RepairedAmount = CASE WHEN TransactionDescription = 'Negative' THEN -1 * Amount ELSE Amount END SELECT TransactionID, Amount, TransactionDescription, SELECT TransactionID, TransactionDescription, Amount INSERT INTO tibbleTransactions(TransactionDescription, Amount) VALUES ('Positive',10),('Negative',20),('Positive',20) try this: use tempdb ĬREATE TABLE tibbleTransactions(TransactionID INT IDENTITY, TransactionDescription VARCHAR(10), Amount INT)
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