Complex Queries and Conditional Logic in PostgreSQL

Welcome back! In our previous lessons, we've explored various ways to query and analyze data using PostgreSQL. We began by mastering text-based queries, moved on to understanding subqueries, and learned to create new columns and perform mathematical operations.

In this lesson, we'll dive deeper into complex queries and conditional logic:

1. Using the CASE statement to categorize data.
2. Combining multiple conditions in the WHERE clause.
3. Enhancing queries with subqueries.

These techniques will help you make your data queries more powerful and insightful.

For convenience, the tables and columns in our Marvel movies dataset are:

Movies Table

 movie_id | movie_name | release_date | phase 
----------+------------+--------------+-------

Movie Details Table

 movie_id | budget_million_usd | box_office_million_usd | imdb_rating | runtime_minutes 
----------+--------------------+------------------------+-------------+-----------------

Characters Table

 character_id | movie_id | character_name | actor | screen_time_minutes 
--------------+----------+----------------+-------+---------------------

The CASE statement allows you to perform conditional logic directly within your queries. It's similar to an IF-THEN-ELSE statement in programming languages.

The syntax of a CASE statement is as follows:

SELECT
    column1,
    column2,
    CASE
        WHEN condition1 THEN result1
        WHEN condition2 THEN result2
        ...
        ELSE resultN
    END AS alias_name
FROM
    table_name;

• CASE: Begins the conditional logic block.
• WHEN condition1 THEN result1: If condition1 is true, the statement returns result1.
• WHEN condition2 THEN result2: If condition2 is true, the statement returns result2.
• ELSE resultN: If none of the conditions are true, the statement returns resultN. This part is optional but recommended for completeness.
• END: Ends the CASE block.
• AS alias_name: (Optional) Provides a temporary name for the resulting column.

Let's see a real-world application of the CASE statement. We'll categorize movies based on their box office earnings into three categories: Flop, Hit, and Blockbuster.