How Groupby Works

Ellen Grant
• Monday, 07 December, 2020
• 7 min read

It allows you to see data in a new way and find key metrics (like the average, maximal, and minimal values in a group of records). In this article, you’ll learn how GROUP BY makes your SQL queries much more powerful and diverse.

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Guest_name origin_cityroom_numberday_inday_outageroom_levelamount_invoicedJuan B. San Pedro10012012-12-282013-01-0732standard$9500Mary J. San Francisco10022013-01-022013-01-1223standard$6700Peter S. Dubai20022013-01-022013-01-2965premium$34000Clair BGenova20012014-07-022014-08-0221standard$16000Meiling Y. San Francisco20022014-11-022014-11-1252standard$9500Olek V. Dubai20032015-01-022015-01-3137premium$28400Benjamin L. San Pedro20022016-01-022016-01-1561premium$15400Arnaldo V. Genova10012017-01-012017-01-0443standard$2500Mary J. San Francisco10022017-01-022017-01-0723standard$4800Wei W. Los Angeles20022018-01-022018-01-2231standard$12000Meiling Y. San Francisco20012018-01-022018-01-2252premium$17500Peter S. Dubai20022019-01-022019-02-2565premium$32000Arnaldo V. Genova20032019-08-052019-08-1743standard$11200Mary J. San Francisco10012019-01-022019-01-1223standard$8900guest_namepreferred_activitycity_namestatecountrycontinentactivityCity_nameStateCountryContinent32Juan B.trekking San PedroAndaluciaSpainEuropeMary J.trekking San FranciscoCaliforniaUnited StatesAmericaPeter S.trekkingDubaiDubaiArabiaAsiaChiara BskiingGenovaLiguriaItalyEuropeMeiling Y.trekking San FranciscoCaliforniaUnited StatesAmericaOlek V.relaxingDubaiDubaiArabiaAsiaBenjamin L.skiing San PedroBuenos AiresArgentinaAmericaWei W.trekking Los AngelesCaliforniaUnited StatesAmericaArnaldo V.skiingGenovaLiguriaItalyEuropeWe want to calculate some statistics, so we can book more guests. We can group records in the table room_guest based on the value of the column origin_city.

The following table shows each group of records in a different color. Guest_name origin_cityroom_numberday_inday_outageroom_levelamount_invoicedPeter S. Dubai20022013-01-022013-01-2965premium$34000Olek V. Dubai20032015-01-022015-01-3137premium$28400Peter S. Dubai20022019-01-022019-02-2565premium$32000Clair BGenova20012014-07-022014-08-0221standard$16000Arnaldo V. Genova10012017-01-012017-01-0443standard$2500Arnaldo V. Genova20032019-08-052019-08-1743standard$11200Wei W. Los Angeles20022018-01-022018-01-2231standard$12000Mary J. San Francisco10022013-01-022013-01-1223standard$6700Mary J. San Francisco10022017-01-022017-01-0723standard$4800Meiling Y. San Francisco20022014-11-022014-11-1252standard$9500Meiling Y. San Francisco20012018-01-022018-01-2256premium$17500Mary J. San Francisco10012019-01-022019-01-1223standard$8900Benjamin L. San Pedro20022016-01-022016-01-1561premium$15400Juan B. San Pedro10012012-12-282013-01-0732standard$9500Now, suppose the hotel’s owner wants to know how many guests come from each city.

In other words, we need the aggregate function COUNT(*), which returns the number of records in a group. COUNT() is a very common function; we’ll return to it later in this article.

It includes a complete description of GROUP BY and several examples of its most common errors. I’d go so far as to say that every SQL query using a GROUP BY clause should have at least one aggregate function.

Metrics are calculated by aggregation functions like COUNT(), SUM(), AVG(), MIN(), and MAX(). However, all of them have something in common: all aggregate functions return a value based on all the records in the group.

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The hotel owner wants to know the maximum value invoiced for each room. In the previous query, we created a report analyzing how much money each room is generating.

Origin_city quantity_of_guestsNULL2Dubai3Genova3Los Angeles1San Francisco5San Pedro2 The WHERE clause is frequently used in SQL queries, so it’s important to understand how it works when combined with GROUP BY. As an example, let's use the previous query, but this time we’ll filter for guests coming from the cities of San Francisco and Los Angeles.

As expected, this result set is shorter than the previous ones; the WHERE clause filtered out many guests, and only the records for rooms in San Francisco and Los Angeles were processed by the GROUP BY clause. Room_number room_levelmax_amount_invoicedmin_amount_ invoicedaverage_amount_invoiced1001standard8900.008900.008900.001002standard6700.004800.005750.002001premium17500.0017500.0017500.002002standard12000.009500.0010750.00 When you’re getting started with GROUP BY, it’s common to run into the following problems.

Let’s look at a similar case where we need to add more than one extra column into the GROUP BY clause. In our data set, we have two different cities named San Pedro, one in Argentina and the other in Spain.

To count these cities separately, we need to group records using the columns city_origin, state, and country. Then we will repeat the first query but add the columns state and country to the GROUP BY clause.

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We also maintain the original COUNT(*) so that the reader can compare both results: Origin_citystatecountrynumber_of_unique_guestsnumber_of_guestsDubaiDubaiUAE23GenovaLiguriaItaly23Los AngelesCaliforniaUnited States11San FranciscoCaliforniaUnited States25San PedroBuenos AiresArgentina11San PedroAndaluciaSpain11Before closing this section, I suggest you watch this 5-minute video on GROUP BY for beginners.

We know the aggregate functions MIN(), MAX(), AVG(), and SUM() compute various statistics. For those readers who want to go a step further, I’ll leave you a link to our SQL Basics course, which covers many interesting topics.

The basic syntax of a GROUP BY clause is shown in the following code block. If you want to know the total amount of the salary on each customer, then the GROUP BY query would be as follows.

Now again, if you want to know the total amount of salary on each customer, then the GROUP BY query would be as follows Its.__STR__() doesn’t give you much information into what it actually is or how it works.

The reason that a DataFrameGroupBy object can be difficult to wrap your head around is that it’s lazy in nature. Groupby (“state”) because it does virtually none of these things until you do something with the resulting object.

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One useful way to inspect a Pandas Group object and see the splitting in action is to iterate over it. If you’re working on a challenging aggregation problem, then iterating over the Pandas Group object can be a great way to visualize the split part of split-apply-combine.

There are a few other methods and properties that let you look into the individual groups and their splits. Each value is a sequence of the index locations for the rows belonging to that particular group.

In the output above, 4, 19, and 21 are the first indices in of at which the state equals “PA.” Groupby () does do some, but not all, of the splitting work by building a Grouping class instance for each key that you pass.

However, many of the methods of the BaseGrouper class that holds these groupings are called lazily rather than at __unit__(), and many also use a cached property design. You can think of this step of the process as applying the same operation (or callable) to every “sub-table” that is produced by the splitting stage.

It simply takes the results of all the applied operations on all the sub-tables and combines them back together in an intuitive way. The dataset contains members’ first and last names, birthdate, gender, type (“rep” for House of Representatives or “sen” for Senate), U.S. state, and political party.

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You can see that most columns of the dataset have the type category, which reduces the memory load on your machine. Now that you’re familiar with the dataset, you’ll start with a “Hello, World!” for the Pandas Group operation.

Groupby () and the comparable SQL statements are close cousins, but they’re often not functionally identical. As we developed this tutorial, we encountered a small but tricky bug in the Pandas source that doesn’t handle the observed parameter well with certain types of data.

In the Pandas version, the grouped-on columns are pushed into the Multitude of the resulting Series by default: This produces a Database with three columns and a Rangefinder, rather than a Series with a Multitude.

In short, using as_index=False will make your result more closely mimic the default SQL output for a similar operation. Also note that the SQL queries above explicitly use ORDER BY, whereas.

If you don't group by City it will just display the total count of Item ID. Analogously, not technically, to keep in mind its logic, it can be thought each grouped field having some rows is put per different table, then the aggregate function carries on the tables individually.

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Ben Fort conspicuously states the following saying. As is DISTINCT keyword, each field specified through GROUP BY is thought as grouped and made unique at the end of the day.

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