## Introduction to Window Functions

SQL is a commonly used programming language and is an option for everyone. In SQL, occasionally you can run into a scenario where you need an answer to a more complicated question than just straight aggregation.

This is where window functions come into play. In this article, we will introduce you to the importance of window functions, the syntax, and the benefits of using templates.

We will then dive into a specific template, Ranking Rows with the RANK() Function.

## Importance of Window Functions

Window functions are a valuable tool for any SQL analyst or developer who wants to do more than just return results. It allows you to define sets of data, also known as a ‘window,’ and then perform calculations over those sets.

This is done without altering the original table, so you’ll have a clearer understanding of data and not affect data in your database.

## Syntax of Window Functions

The window function consists of a function call, an expression that defines the data that will be processed, a partition list to define the window specification syntax, and an optional order list and window frame clause. The function call is the window function, and it is followed by its arguments.

The arguments describe the data you want to use to calculate the results of the window function. The expression defines the specific column that you want to perform the calculation on.

## Benefits of Using Templates

For SQL beginners, templates are valuable resources that will guide them when starting a query. Using templates help you write down your thoughts in advance and save time in the code editor as you have pre-written codes to use.

It also keeps you focussed on the problem you are trying to solve. Template 1: Ranking Rows with the RANK() Function

To illustrate the application of window functions and templates, we will use the RANK() function to rank the rows of a table.

Suppose we have a table called rich_people with columns id, first_name, last_name, and net_worth. We would like to rank the top 5 richest people in this table.

## Using RANK() Function

RANK() is an example of a ranking function in SQL that ranks the rows in a result set based on a column’s value. The RANK() function assigns each record value to a unique rank value.

Since this function considers duplicates, multiple rows get the same number of ranks.

Example Table: rich_people

We will use a table called rich_people to explain how to apply the template.

The table has four columns: id, first_name, last_name, and net_worth. The table contains personal information about five people regarding their net worth.

## Syntax for Ranking

To calculate the ranks in the table, we can use the following syntax:

```
SELECT id, first_name, last_name, net_worth,
RANK() OVER (ORDER BY net_worth DESC) as rank
FROM rich_people;
```

## In this syntax:

- We use the SELECT statement to select the columns we need from the table.
- We use the RANK() function to generate the ranks.
- The ORDER BY clause is optional but recommended as it specifies the ordering of the net_worth values. In this case, we want to order them in descending order to get the richest people first.
- We use the OVER() clause to define the window specification for the RANK function.

## Result and Usage

After executing the query with the given syntax, we would get the top 5 people according to their net worth. The result table will show the columns id, first_name, last_name, net_worth, and rank.

With this information, we can answer questions about who the richest people in the rich_people table are and identify individuals who meet specific conditions.

## Conclusion

In conclusion, we have provided an overview of window functions and how they can be applied in SQL. We have shown how working with templates can save time and help focus on the problem at hand.

With the RANK() function, we have demonstrated how ranking rows in a result set can give us insights into our data and help us answer questions more effectively. By incorporating window functions into everyday SQL queries, we can make our data more insightful and turn raw data into useful information.

## Template 2: Difference Between Two Consecutive Periods

In time series data, one of the most frequently asked questions is how to calculate the difference between two consecutive periods. By finding the difference between two consecutive periods, we can measure growth or decline, make informed decisions, and identify trends.

In this second template, we will use the LAG() function to calculate the difference between two consecutive periods. We will take a look at an example table, country_gdp, and use the table to illustrate how to calculate differences between the same value for each year.

### Calculating Daily/Monthly/Quarterly Differences

The LAG() window function makes it easy to create a sequence of time periods. The function retrieves previous rows’ values based on the order then subtracting the current row value from the previous row value returns the difference.

The difference can be used to find the change in values over time, like daily, monthly, quarterly, or yearly differences. Example Table: country_gdp

Let’s use a table called country_gdp to explain how to apply the template.

The table has three columns: country, year, and GDP. The table contains information about the GDP of different countries for three years.

## Syntax for Calculating Differences

To calculate the difference between the same value for each row of the year column, we can use the following syntax:

```
SELECT country, GDP, year,
GDP - LAG(GDP) OVER (PARTITION BY country ORDER BY year) AS GDP_difference
FROM country_gdp;
```

## In this syntax:

- We use the SELECT statement to select the columns we need from the table.
- We subtract the GDP value from the LAG value across the country and year by using the OVER() clause to define the window function.
- We use the PARTITION BY clause to group GDP values by country.
- We use the ORDER BY clause to sort the year values.

## Result and Usage

After executing the query with the given syntax, we would finally have the difference in GDP between two consecutive periods. With this information, we can answer questions such as whether the GDP increased or decreased over time, the percentage of increase/decrease, and identify potential issues with certain time periods.

## Template 3: Percent Difference Between Two Consecutive Periods

In template 3, we’ll look at how to calculate the percentage difference of values between two consecutive periods. This calculation enables us to examine trends over time, and it’s useful for extracting insights from financial data.

We’ll use the eur_usd table, which contains the daily values of the Euro to the US Dollar exchange rate for a defined date range.

## Variation of Template 2

In Template 2, we calculated the difference between values of two consecutive periods. In Template 3, we will be taking this one step further and calculating percentage differences between consecutive periods.

We’ll achieve this by using a moving average, also known as a rolling mean or rolling average. This approach calculates the average of a set of numbers over a specified period, which then shifts to the next period to calculate a new average.

This enables us to calculate the differences between consecutive periods. Example Table: eur_usd

In this example, let’s say we have a table called eur_usd, which tracks the exchange rate between Euro and the US Dollar over some time.

The table has two columns: date and exchange_rate.

## Syntax for Calculating Percent Differences

## We can calculate the percentage difference between two consecutive exchange rate periods by using the following syntax:

```
SELECT date, exchange_rate,
ROUND(((exchange_rate - LAG(exchange_rate) OVER (ORDER BY date)) / LAG(exchange_rate) OVER (ORDER BY date) * 100), 2)
AS exchange_rate_percent_difference
FROM eur_usd;
```

## In this syntax:

- We use the SELECT statement to select the columns we need from the table.
- We use the ROUND() function to round the calculated percentage to two decimal places.
- We use LAG() function to retrieve the previous exchange rate value per date and calculate the percentage difference between the current exchange rate value and the previous value.

## Result and Usage

After executing the query with the given syntax, we would finally have the percentage difference between two consecutive exchange rates. With this information, we can answer questions such as the exchange rate’s percentage increase or decrease over time or identify patterns in the data.

## Conclusion

With Templates 2 and 3, we have seen how we can calculate differences between consecutive periods and percentage differences with the help of window functions. Calculating differences help us extract valuable insights by identifying trends or highlighting issues.

By incorporating these techniques into our SQL queries, we can obtain vital information from large datasets, enabling us to make more informed and better business decisions. Template 4: Calculating the Running Total

A running total is a cumulative measure of a numerical value that changes sequentially.

It is a valuable tool for tracking the average, totals, and growth of various aspects over time. In this template, we will use the SUM() function to find the running total in a table.

We will take a look at an example table named subscribers, which contains information on the number of new subscribers per month in different cities. Example Table: subscribers

Suppose we want to calculate the running total of the new subscribers per month for various cities.

In this case, we can use a table called subscribers. This table will have three columns: city, new_subscribers, and month.

## Syntax for Calculating Running Total

To calculate the running total of new subscribers for each city, we can use the following syntax:

```
SELECT city, month, new_subscribers,
SUM(new_subscribers) OVER (PARTITION BY city ORDER BY month ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
AS cumulative_subscribers
FROM subscribers
ORDER BY city, month;
```

## In this syntax:

- We use the SELECT statement to select the columns we need from the table.
- We use the SUM() function to calculate the sum of new subscribers for each city.
- We use the OVER() clause to define the window function.
- We use the PARTITION BY clause to group the data by city.
- We use the ORDER BY clause to order the data by month.
- We use ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW to specify the range of rows to sum up, which is from the oldest to the most recent data.

## Result and Usage

After executing the query with the above syntax, we will get a result set that shows the city, the month, the number of new subscribers, and the cumulative total for each city. By applying the running total, we can see which cities have been adding new subscribers over time and which cities have not been adding many.

For example, suppose we want to find out how many new subscribers were added in each city from the beginning of the period through April. In that case, we could look at the cumulative_subscribers column for each city and then subtract the oldest cumulative_subscribers from the cumulative_subscribers in April to get the results.

## Conclusion

In conclusion, we have demonstrated the use of a running total in SQL to summarize data over time. By understanding how to calculate the running total in SQL queries, we can generate valuable insights to inform strategic business decisions.

The SUM() function used in conjunction with the OVER() clause can help create cumulative sums that iteration through partitions based on a given column or set of columns in order to have access to context-based data while performing the calculations. We hope that this template proved to be useful for your data analysis needs.

In this article, we explored the importance of window functions in SQL and provided examples of how to use them to analyze time series data. We covered three templates that showed how to calculate differences between consecutive periods, percent differences, and a running total.

By utilizing these templates, we can extract valuable insights from large datasets that can help inform strategic business decisions. Window functions are incredibly useful for anyone looking to analyze time series data in SQL.

Incorporating them into everyday queries can make our data more insightful and turn raw data into useful information, providing vital insights that enable us to make informed decisions.