CASE
POSTGRESQL

PostgreSQL CASE: A Comprehensive Guide

intro

Let’s explore the PostgreSQL CASE function. In this article, we’ll take a look at what it is, the ins and outs, and why it is important in database management systems. Enjoy the read!

Tools used in the tutorial
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Dbvisualizer DBVISUALIZER
TOP RATED DATABASE MANAGEMENT TOOL AND SQL CLIENT
PostgreSQL logo POSTGRESQL
THE POSTGRESQL DATABASE

Known for its scalability, and extensibility, PostgreSQL is a potent open-source relational database management system. Due to its extensive feature set, PostgreSQL is a preferred option for DBAs and developers. The CASE statement, one of PostgreSQL's features, enables conditional logic and data manipulation in SQL queries. This guide will provide an understanding of its syntax and its applications in forming conditional queries.

What Is the PostgreSQL CASE Command?

In PostgreSQL, you may do conditional evaluations and return different values depending on predefined conditions. That is what the CASE command is all about.  In detail, that functions as a  conditional expression tool for managing the flow of queries and also altering data.

Whether you need to do basic comparisons or more intricate logical processes, the PostgreSQL CASE statement is the right tool.

How To Use CASE in PostgreSQL

Let’s look at the general syntax of PostgreSQL CASE and its basic usage:

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1 CASE 2 WHEN condition1 THEN result1 3 WHEN condition2 THEN result2 4 ... 5 ELSE result 6 END;

When a condition evaluates to false, the CASE expression evaluates the next condition from top to bottom until it finds a condition that is true.

If a condition evaluates to true, the CASE expression returns the corresponding result that follows the condition. It immediately stops evaluating the next action.

In case all conditions evaluate to false, the CASE expression returns the result that follows the ELSE keyword. If you omit the ELSE clause, the CASE expression returns NULL.

The PostgreSQL CASE statement begins with CASE and is followed by one or more WHEN clauses, each specifying a condition and the corresponding result value. The ELSE clause is optional and provides a default result when none of the conditions are met.

Now, we know the basic syntax. It is important to know that the CASE statement has two forms. There’s the simple CASE form and there’s the searched CASE form. Time to learn more!

Simple CASE statement

The simple CASE statement compares a single expression to multiple values and returns a result based on the matching value. This is a concise way of expressing multiple equality conditions.

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1 CASE expression 2 WHEN condition_1 THEN result_1 3 WHEN condition_2 THEN result_2 4 ... 5 WHEN value_n THEN result_n 6 ELSE else_result 7 END;

This is what occurs in the statement:

  • CASE initiates the CASE statement.
  • WHEN clause defines a condition. If the condition is true, the result is returned.
  • The ELSE clause (optional) provides a default result if none of the previous conditions are met.
  • END concludes the CASE statement.

Now, build a query that will obey the simple CASE form.

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1 SELECT product_name, 2 CASE product_category 3 WHEN 'Electronics' THEN 'High-Tech' 4 WHEN 'Clothing' THEN 'Fashion' 5 WHEN 'Furniture' THEN 'Home Decor' 6 ELSE 'Other' 7 END AS category_group 8 FROM products;

In this example, the simple CASE statement compares the value of the product_category column and assigns a corresponding category_group.

Searched CASE statement

With the searched CASE statement, multiple conditions are evaluated independently, allowing for more complex evaluations and expressions. Look at a clear example of this form of CASE statement.

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1 SELECT order_id, order_quantity, 2 CASE 3 WHEN order_quantity > 100 THEN 'Large' 4 WHEN order_quantity > 50 THEN 'Medium' 5 ELSE 'Small' 6 END AS order_size 7 FROM orders;

In this example, the searched CASE statement evaluates the order_quantity column and assigns an order size based on the specified conditions.

Why Use the PostgreSQL CASE Statement?

Here are some of the reasons why you might consider using the PostgreSQL CASE statement in your database development journey:

  1. Custom sorting: Define custom sorting orders for result sets. You can assign different sort orders based on specific conditions, providing more control over the ordering of query results.
  2. Handling NULL values: Handle NULL values and provide default values or alternative results. It allows you to replace NULL values with meaningful values or perform specific actions based on the presence of NULL values.
  3. Conditional transformations: Transform data based on specific conditions. You can use it to categorize data into different groups, making it easier to analyze and understand the data.

CASE in PostgreSQL: Use Cases

Time to explore some of the potential use cases of the CASE statement in PostgreSQL using the sample table below.

Sample Table in DbVisualizer
Sample Table in DbVisualizer

Data Categorization

Suppose we want to categorize names of customers based on specific conditions - their total purchases based on the following logic:

  • If the total purchase is greater than 1000, then the customer name should be categorized under “VIP”.
  • If the total purchase exceeds 500, the customer name should be categorized under “Preferred”.
  • If the total purchase is less than 500, then “Regular”.

To apply this logic, you can use the CASE expression in the SELECT statement as follows:

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1 SELECT customer_name, 2 CASE 3 WHEN total_purchases > 1000 THEN 'VIP' 4 WHEN total_purchases > 500 THEN 'Preferred' 5 ELSE 'Regular' 6 END AS customer_category 7 FROM customers;

This is the result of the SQL query in the PostgreSQL database client DbVisualizer:

Query Output in DbVisualizer
Query Output in DbVisualizer

We can see in the image above that the result aligns correctly with the logic behind the SQL query.

Custom Sorting

The CASE statement can also be used to define custom sorting orders for result sets. For example, sorting employees based on their job titles:

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1 SELECT employee_name, job_title 2 FROM employees 3 ORDER BY 4 CASE job_title 5 WHEN 'Manager' THEN 1 6 WHEN 'Supervisor' THEN 2 7 ELSE 3 8 END;

Custom Sorting Output in DbVisualizer
Custom Sorting Output in DbVisualizer

Here again, we can see in DbVisualizer above that the result aligns correctly with the logic behind the SQL query.

Handling Null Values

The CASE statement can also be used to handle NULL values and provide default values or alternative results. Write a query that will replace NULL values with “0”:

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1 SELECT product_name, price, 2 CASE 3 WHEN price IS NULL THEN '0' 4 ELSE price 5 END AS formatted_price 6 FROM products;

Null Values Handled in DbVisualizer
Null Values Handled in DbVisualizer

Best Practices for Using the PostgreSQL CASE Statement

When using the PostgreSQL CASE statement, it's important to follow certain best practices to ensure readable and systematic code.

  1. Use comments for clarity: If your CASE statement involves complex conditions or transformations, add comments to explain the logic. This will help other developers understand the flow and logic of the code.
  2. Testing: In production, before using a CASE statement, thoroughly test it and validate it with different use cases.
  3. Order conditions appropriately: In a simple CASE statement, the conditions are evaluated based on the sequence of the query, and the first condition is always executed first. Ensure that the conditions are ordered appropriately to achieve the desired results.

Conclusion

In this article, you learned that CASE is a powerful tool for conditional logic and data manipulation within SQL queries.

We began by grasping the rationale behind the PostgreSQL CASE statement — in basic comparison and intricate logical processes. This streamlines conditional evaluations and returns different values based on specified conditions, acting as a versatile tool for transforming data and controlling the flow of queries.

To better appreciate its capabilities, you need a tool that helps you manage databases and visually explore query results. This is where a full-featured database client like DbVisualizer comes in. In addition to being able to connect to several DBMSs, it offers advanced query optimization functionality, and full support for all PostgreSQL features, including CASE. Download DbVisualizer for free now!

FAQs

What is the difference between a simple CASE statement and a searched CASE statement in PostgreSQL?

The conditions they handle are where the primary differences reside. A CASE statement that is searched permits more complicated criteria in each WHEN clause, whereas a simple CASE statement compares a single expression to several possible values through equality.

Can I use the PostgreSQL CASE statement in conjunction with other SQL clauses, such as WHERE or UPDATE?

Yes, the PostgreSQL CASE statement is versatile and can be used in various SQL clauses. For example, you can use it in SELECT clauses to conditionally transform data, in WHERE clauses to filter rows based on conditions, or in UPDATE clauses to modify data based on specified criteria.

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About the author
Leslie S. Gyamfi.
Leslie S. Gyamfi
Leslie Gyamfi is a mobile/web app developer with a passion for creating innovative solutions. He is dedicated to delivering high-quality products and technical articles. You can connect with him on LinkedIn
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