Emil Drkusic
SQL Server PIVOT TABLE - the data model we'll use

Learn SQL: SQL Server Pivot Tables

May 11, 2020 by

In the previous few articles in this series, we’ve set the foundations on how to create a report. We’ll take one step further and see how to use the SQL Server PIVOT table operator. We’ll start from the simple query and slowly progress towards dynamic SQL and PIVOT. Let’s start.

Data model and the general idea

The data model we’ll use is the same one we’ve been using throughout this series.

SQL Server PIVOT TABLE - the data model we'll use

Our task today is to create a report (using SQL Server PIVOT operator) where each city will be in a separate row, and where we’ll count the number of all call outcomes related to each city. Therefore, all outcomes (all values from the dictionary) shall be columns in our report.

Since “a picture is worth a thousand words”, we’ll describe our final goal with the picture too.

The result we want

To achieve this, we’ll need to use data from 4 tables call, call_outcome, customer, and city. Therefore, it would be good that you take a closer look at the model below and analyze how these tables are related.

We’ll start by examining the data in the tables and, with each query, move one step closer to the desired result (query). The approach used here is the same as the one used in Learn SQL: Create a report manually using SQL queries article.

Report categories and data

The first set of queries we’ll start with is the one examining data currently present in all 4 tables we’ll need to use to create the report. We’ll be doing it so we can confirm that the final report returned what it should. After that, we’ll create queries which return reporting categories and the report data, as well as SQL Server PIVOT table queries.

Reporting data in the database tables

These queries are as simple as they could be, so there is nothing special to comment regarding their syntax. Regarding the data returned, we should be aware of the following:

  • The city table contains 6 cities, and therefore we should have 6 rows in the final report
  • There are 3 possible outcomes in the call_outcome table, so we should have 3 columns for outcomes (4 columns total -> one is for city name)
  • The table call has 10 rows (only 8 on the picture above), so in the final table, the sum of all outcomes should be 10 (each call can have exactly 1 outcome)

The next thing to do is to prepare reporting categories. We want to have a combination of all cities and all possible call outcomes. To achieve that, we’ll use the CROSS JOIN (Cartesian product).

Report categories

Having all the reporting categories shall guarantee that we’ll have a row in the report, no matter if that pair has data or not. And that is what we want – to see 0 in that reporting category, and not to miss that category entirely.

The next thing to do is joining all 4 tables containing the data we need.

Report data

The query itself is not complex. In the result, we should notice that we have 10 rows, the same number as the number of calls we have in the database. Also, I’ve used the DATEDIFF function to return the duration of each call in seconds. I could simply put call.id here, but I wanted to remind ourselves of the DATEDIFF function we’ve mentioned in this article. We could use this duration if we need a SUM/AVG of seconds spent per each reporting category (city & call outcome).

Report without SQL Server PIVOT TABLE

Now we’re ready to join categories and data. We’ll use both previously mentioned queries as subqueries and join them using LEFT JOIN (in order to have all the categories present in the final output).

Report without PIVOT

We have all 18 combinations (6 different cities * 3 different call outcomes), and we also have all 10 calls present here (rows with the call duration <> NULL).

Let’s now create the SQL Server PIVOT query.

SQL Server PIVOT TABLE (static)

So far, we’ve managed to pull the data we need, and we have them as the list. We could export data to Excel and make transformations there. We’ll actually do this in the upcoming article. Still, today, we want to solve this using SQL Server PIVOT operator. In this section, we’ll cover the “static” SQL Server PIVOT. Let’s take a look at the query used and the result first.

Static SQL Server PIVOT query

We can notice that the result is exactly what we wanted. We have each city in one row and all 3 categories for call outcomes in separate columns. If there is no data for a certain city-outcome pair, that cell shall contain the value 0.

Let’s comment on the SQL Server PIVOT query now. These are a few things I find important to mention here:

  • In the first part of the query – report_data, we’ve copy-pasted the query from the previous section. The only change was that we had no ORDER BY and GROUP BY in this part of the query because they can’t be applied here (they should go after the final query result is generated)
  • The PIVOT part of the query consists of 2 parts. In the first, we’ll define which aggregate function we want to apply. In our case, this is – COUNT(call_duration). In FOR part of the query, we’ll define columns. We literary list all the values we want to have as columns. This is hard-coding. If we add new value to the table, that won’t impact the query (and it should)

This query does its job, and it will work perfectly if we never change (add new, delete, update their names) call outcomes in the related table. That could prove to be the problem in cases we need to make changes in the dictionary. We don’t want to think if there is a query that doesn’t work as expected. To solve this, we’ll need to use a new concept – dynamic SQL.

Dynamic SQL Server PIVOT TABLE

Before moving to the code, let’s shortly explain what the dynamic SQL really is (we’ll give a much more detailed review of it in upcoming articles). The simplest explanation is that in SQL, you can “build” your queries as strings, and then pass that string as a parameter for the SQL Server stored procedure. This procedure shall execute the SQL statement(s) stored in that string (of course, if the syntax is OK).

While this might sound like something not so commonly used, it has quite a few places where it makes your life much easier. Creating a SQL Server PIVOT table query with an unknown number of columns is exactly one such case.

The idea of this approach is as follows:

  • We’ll declare a variable where to store all column names (@columns), and the variable where to store the complete query (@query)
  • Using the SELECT query, we’ll find all values stored in the outcome_text column. The following code – @columns += QUOTENAME(TRIM(co.outcome_text)) + ‘,’, shall append column name to the list of all previous column names returned by the query. The result is that we have all column names stored in the variable @columns
  • In the @query variable, we’ll store the complete query from the previous section, except for the part where columns are defined. We’ll get this part from the variable @columns
  • The last thing we need to do is to pass the complete query as the parameter to the SQL Server system procedure sp_executesql

SQL Server PIVOT TABLE - dynamic SQL query

The final result is the same as in the previous section, but this time we’re sure our query will work even if we make changes to the outcome.outcome_text values. Also, you can easily modify this query and calculate any other values, e.g., SUM/AVG call duration per city-outcome pair.

Conclusion

The SQL Server PIVOT operator gives you a completely new view of what you can achieve directly on the database layer. When combined with the dynamic SQL, this goes even further. I strongly encourage you to play with it – there is no better way of learning than to try it yourself. In the next article, we’ll show how we could use SQL query output and create tables and graphs in Excel (this shall work not only in SQL Server but generally).

Table of contents

Learn SQL: CREATE DATABASE & CREATE TABLE Operations
Learn SQL: INSERT INTO TABLE
Learn SQL: Primary Key
Learn SQL: Foreign Key
Learn SQL: SELECT statement
Learn SQL: INNER JOIN vs LEFT JOIN
Learn SQL: SQL Scripts
Learn SQL: Types of relations
Learn SQL: Join multiple tables
Learn SQL: Aggregate Functions
Learn SQL: How to Write a Complex SELECT Query?
Learn SQL: The INFORMATION_SCHEMA Database
Learn SQL: SQL Data Types
Learn SQL: Set Theory
Learn SQL: User-Defined Functions
Learn SQL: User-Defined Stored Procedures
Learn SQL: SQL Views
Learn SQL: SQL Triggers
Learn SQL: Practice SQL Queries
Learn SQL: SQL Query examples
Learn SQL: Create a report manually using SQL queries
Learn SQL: SQL Server date and time functions
Learn SQL: Create SQL Server reports using date and time functions
Learn SQL: SQL Server Pivot Tables
Learn SQL: SQL Server export to Excel
Learn SQL: Intro to SQL Server loops
Learn SQL: SQL Server Cursors
Learn SQL: SQL Best Practices for Deleting and Updating data
Learn SQL: Naming Conventions
Learn SQL: SQL-Related Jobs
Learn SQL: Non-Equi Joins in SQL Server

Emil Drkusic
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SQL commands, T-SQL

About Emil Drkusic

Emil is a database professional with 10+ years of experience in everything related to databases. During the years, he worked in the IT and finance industry and now works as a freelancer. His past and present engagements vary from database design and coding to teaching, consulting, and writing about databases. Also not to forget, BI, creating algorithms, chess, philately, 2 dogs, 2 cats, 1 wife, 1 baby... You can find him on LinkedIn View all posts by Emil Drkusic

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