# Mastering Data Analysis: The Power of GROUP BY and PARTITION BY in SQL

## Data Analysis with GROUP BY and PARTITION BY

Data analysis is a critical aspect of modern-day business. In pursuit of meaningful insights, data analysts employ different tools to manipulate and extract data from large datasets.

Among the most useful tools are GROUP BY and PARTITION BY, which enable them to group data based on given criteria, perform calculations on multiple levels of aggregation, and transform the data into new result sets.

### 1) GROUP BY Clause

GROUP BY is a powerful SQL clause that groups data based on specific criteria. It is an essential tool for summarizing data and extracting insights from large datasets. The clause groups data based on one or more columns, which provides a structure for data analysis.

In simple terms, the GROUP BY clause takes a large dataset and breaks it down into smaller, more manageable subsets. The criteria for grouping data can range from simple categorization to complex calculation.

GROUP BY can categorize data based on simple criteria like name or city or perform calculations on data. Typically, the GROUP BY clause works in tandem with an aggregate function, which performs calculations on the grouped data.

Aggregate functions can calculate sums, averages, minimum, or maximum values, among other calculations. Example of GROUP BY query and result:

Consider a table named sales that contains data on sales made by a company. The table has columns like date, name, product, price, and quantity. We want to group sales by date and product, showing the total sales made for each product on a given date.

### The query looks like this:

``` SELECT date, product, SUM(price * quantity) AS total_sales FROM sales GROUP BY date, product ```

In this query, we group the data in the sales table by date and product. We then calculate the total sales for each group based on the price and quantity of the product sold.

The result is a table that shows the total sales made for each product on a given date. Use of subqueries to combine original row-level details with aggregated values from GROUP BY:

SQL subqueries are a powerful tool for combining data from multiple tables or for combining row-level details with aggregate values.

Subqueries allow analysts to group data, summarize it, and then combine the summarized data with the original data. This provides a more comprehensive view of the data and allows analysts to extract more meaningful insights.

Consider the example above, where we used the GROUP BY clause to calculate total sales for a given date and product. If we want to include additional data on each sale, such as the name of the salesperson who made the sale, we can use a subquery to combine the original row-level data with the aggregate values.

### The query would look like this:

``` SELECT s.date, s.product, s.salesperson, s.price, s.quantity, t.total_sales FROM sales s INNER JOIN (SELECT date, product, SUM(price * quantity) AS total_sales FROM sales GROUP BY date, product) t ON s.date = t.date AND s.product = t.product ```

In this query, we use a subquery to calculate the total sales for each product on a given date. We then join the subquery result with the sales table to include the original row-level data on each sale.

The result is a table that shows the total sales made for each product on a given date, along with additional details like the name of the salesperson who made the sale. PARTITION BY is another powerful SQL clause that is closely related to GROUP BY.

### 2) PARTITION BY Clause

PARTITION BY is used in combination with the OVER() clause to perform calculations on multiple levels of aggregation. While GROUP BY groups data based on specific criteria, the PARTITION BY clause divides data into partitions based on one or more columns.

This division enables analysts to perform multiple calculations on different sets of data in a single query. Example of PARTITION BY query and result:

Consider a table named sales that contains data on sales made by a company. The table has columns like date, name, product, price, and quantity. We want to calculate the total sales made by each salesperson, as well as their percentage of total sales.

If we only use GROUP BY, we can only calculate total sales per salesperson and not their percentage of total sales. We can use the PARTITION BY clause to partition the data by the salesperson column and then use the SUM() window function to calculate the sum of total sales for each salesperson.

We can then use the SUM() function again, this time partitioned by all salespeople, to calculate the total sales across all salespeople. The query looks like this:

``` SELECT salesperson, SUM(total_sales) AS total_sales, SUM(total_sales)/SUM(SUM(total_sales)) OVER () * 100 AS percentage_of_total_sales FROM (SELECT salesperson, SUM(price * quantity) AS total_sales FROM sales GROUP BY salesperson, date) t GROUP BY salesperson ```

In this query, we first group the data in the sales table by salesperson and date. We then calculate the sum of total sales for each salesperson using the PARTITION BY clause and the SUM() window function.

We then use the same SUM() function, this time partitioned by all salespeople, to calculate the total sales across all salespeople. We then use the sum of sales per salesperson and the total sales across all salespeople to calculate the percentage of total sales for each salesperson.

### Comparison of similarities and differences between GROUP BY and PARTITION BY:

GROUP BY and PARTITION BY are similar in that they both allow for grouping of data based on specific criteria. The two clauses allow analysts to perform calculations on data subsets and then view the results in a structured format.

Both GROUP BY and PARTITION BY can perform aggregate functions on data subsets, making them valuable analytical tools. The key difference between GROUP BY and PARTITION BY is that the GROUP BY clause groups data based on specific columns, while the PARTITION BY clause divides data into partitions based on one or more columns.

This division enables analysts to perform multiple calculations on different sets of data in a single query. In contrast, GROUP BY only allows for a single level of aggregation.

### Conclusion:

GROUP BY and PARTITION BY are two of the most powerful tools in SQL for data analysis. They provide a structured format for viewing data subsets, performing aggregate functions on data, and extracting insights from large datasets.

While the two clauses are similar, they have key differences that make them valuable for different analytical tasks. By understanding the differences between the two clauses, analysts can choose the appropriate tool for their specific use case and extract meaningful insights from their data.

### 3) Example Query with Train and Journey Tables

The tables Train and Journey contain data on train schedules and routes, respectively. The Train table has columns like train_id, departure_date, and departure_time, while the Journey table has columns like train_id, start_station, end_station, duration, and distance.

The Train table has one row for each train, while the Journey table has one row for every journey a train makes. Example GROUP BY query for aggregated values of routes:

We want to group the data in the Journey table by the start_station and end_station columns to determine the number of journeys per route.

### The query looks like this:

``` SELECT start_station, end_station, COUNT(*) AS num_journeys FROM Journey GROUP BY start_station, end_station ```

In this query, we group the data in the Journey table by the start_station and end_station columns. We then use the COUNT() function to count the number of journeys for each route.

The result is a table that shows the number of journeys for each route. Results of aggregated values and loss of original row-level details:

The GROUP BY clause enables analysts to aggregate data based on specific criteria, but it has a downside. Aggregating data results in the loss of original row-level details. When we group data using GROUP BY, we do not retain the individual row-level details that make up the grouped data.

In our example, we only get the number of journeys per route. We cannot tell from the result which trains made the journeys or when they occurred.

### 4) PARTITION BY Clause

The PARTITION BY clause is a powerful extension of the GROUP BY clause that enables analysts to perform calculations on multiple levels of aggregation while preserving original row-level details. The PARTITION BY clause divides data into partitions based on one or more columns, independently of other partitions.

This division enables analysts to perform multiple calculations on different sets of data in a single query. Definition and use of PARTITION BY with OVER() and window functions:

The PARTITION BY clause works in tandem with the OVER() clause, which defines the set of rows to which a window function is applied.

The window function operates on each partition created by the PARTITION BY clause. The OVER() clause consists of two parts: the PARTITION BY clause that divides the data into partitions, and the ORDER BY clause that specifies the order of the rows within each partition.

Consider the following query, where we calculate the running total of distance traveled by each train, partitioned by train_id:

``` SELECT train_id, departure_date, departure_time, start_station, end_station, distance, SUM(distance) OVER(PARTITION BY train_id ORDER BY departure_date, departure_time) AS running_distance FROM Journey INNER JOIN Train ON Journey.train_id = Train.train_id ```

In this query, we use the PARTITION BY clause to partition the data by train_id and the ORDER BY clause to order the rows in each partition by departure_date and departure_time. We then use the SUM() window function to calculate the running total of distance traveled by each train for each row.

The result is a table that shows the running distance traveled by each train at each stop. Preservation of original row-level details with PARTITION BY:

The main advantage of the PARTITION BY clause over the GROUP BY clause is that it preserves original row-level details.

Because the PARTITION BY clause divides data into partitions independently of other partitions, the result of the query is a table that shows the aggregated values alongside the original row-level data. In our example query, we can tell which train traveled which route and when they did it.

### Comparison of PARTITION BY and GROUP BY:

While the GROUP BY and PARTITION BY clauses may seem similar, they operate differently. GROUP BY groups data based on specific columns and aggregates the data, reducing the number of rows in the result set.

In contrast, PARTITION BY divides the data into partitions based on one or more columns while retaining the original rows. When we use PARTITION BY, we can perform calculations on each partition while retaining detail information about each row.

Use of standard aggregate functions as window functions with OVER():

Standard aggregate functions like SUM(), AVG(), COUNT(), MIN(), and MAX() can be used as window functions with OVER(). When used as a window function, the standard aggregate functions calculate the aggregate for each row in the partition instead of the whole partition.

### Description of other important window functions:

There are several other important window functions in SQL, including ROW_NUMBER(), RANK(), DENSE_RANK(), NTILE(), LEAD(), and LAG(). These functions enable analysts to perform complex calculations on data subsets or compare data across different rows.

For example, the ROW_NUMBER() function assigns a unique number to each row in a partition. The RANK() function assigns a rank to each row in a partition, with ties receiving the same rank.

The DENSE_RANK() function assigns a rank to each row in a partition, without any gaps in the rank values. The NTILE() function divides a partition into a specified number of tiles and assigns each row to a tile based on the order specified by the ORDER BY clause.

The LEAD() and LAG() functions allow analysts to access values from other rows within the partition, either before or after the current row. In conclusion, the GROUP BY and PARTITION BY clauses are powerful SQL tools for data analysis.

While GROUP BY is useful for simple aggregations, the partitioning capability of the PARTITION BY clause allows for more advanced aggregation and complex calculations while retaining the original row-level details. By understanding the differences between these clauses and the window functions that can be used with them, analysts can effectively extract meaningful insights from large datasets.

### 5) Similarities and Differences between GROUP BY and PARTITION BY

GROUP BY and PARTITION BY are both essential SQL clauses used for data analysis. They allow analysts to group and aggregate data based on specific criteria, which can aid in the extraction of valuable insights from large datasets.

### Summary of similarities and differences between GROUP BY and PARTITION BY:

The main similarity between GROUP BY and PARTITION BY is that both clauses group data based on specific criteria. GROUP BY groups data based on one or more columns, while PARTITION BY divides data into partitions based on one or more columns.

The similarity between them stops at the aggregation level.

The main difference between GROUP BY and PARTITION BY is that GROUP BY performs aggregation at a single level, while PARTITION BY performs aggregation at multiple levels.

The GROUP BY clause reduces the number of rows in the result set by grouping multiple rows based on specified columns, whereas the PARTITION BY clause separates a dataset into multiple partitions based on the specified columns without reducing the number of rows. By doing so, PARTITION BY can perform calculations on different levels of aggregation within the same query, retaining the original row-level details.

### Use of GROUP BY in cases when PARTITION BY would be better:

In situations where data needs to be grouped together and have specific calculations performed on each group, GROUP BY is a viable choice. Still, if calculations must be performed based on levels of aggregation, or the entire dataset must be divided into subsets, we can use PARTITION BY.

In other words, GROUP By is suited for single level aggregations, while PARTITION BY is better for Multilevel. Use of subqueries with GROUP BY to simulate PARTITION BY:

If we do not have the privilege of utilizing the PARTITION BY clause, we can simulate it using subqueries.

A subquery works by nesting the query that produces the aggregate columns within another query. Subqueries can concatenate the original row-level details with the aggregate values derived from the GROUP BY clause to generate a new table consisting of both summarized and original data.

For example, suppose we have multiple rows of data with different details and want to aggregate the results based on specific criteria. In that case, we can use GROUP BY in conjunction with a subquery to simulate the PARTITION BY functionality.

Consider a scenario where we have a table with sales data containing multiple rows of data with different details like date, product type, and quantity. We want to aggregate the data by product type and also calculate the total number of units sold and the highest selling product for each product type.

This requires combining the original row-level details with the aggregate values derived from the GROUP BY clause. The subquery would look like this:

``` SELECT product_type, MAX(total_units_sold) AS units_sold FROM (SELECT product_type, product, SUM(quantity) AS total_units_sold FROM sales GROUP BY product_type, product) AS sub_query GROUP BY product_type ```

In this subquery, we group the sales data by the product type and product and calculate the total number of units sold for each product. We then use the MAX() aggregation function to calculate the highest number of units sold for each product type.

Finally, we group the result by the product type to retrieve the total number of units sold and the highest-selling product that meets the required criteria. Conclusion:

GROUP BY and PARTITION BY are both essential SQL clauses for data analysis, each with its unique features and use cases.

While GROUP BY is better suited for single-level aggregation, PARTITION BY allows analysts to divide datasets into multiple partitions based on the specified columns, enabling multilevel aggregation while preserving the original row-level details. Although there are no explicit keyword-based solutions for simulating PARTITION BY functionalities in SQL, we can utilize subqueries to address this problem to a degree.

Ultimately, understanding the differences between the two clauses allows analysts to choose the appropriate tool for their specific use case and extract valuable insights from large datasets.