SQL: Understanding Case Statements

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When working with SQL, one of the most common concepts is the use of case statements to make decisions based on certain conditions. However, many developers struggle to understand how to properly implement these statements in their queries. In this article, we’ll delve into the world of SQL case statements and explore why some developers might run into issues with them.

ER Diagram: Understanding the Problem

The problem presented in the Stack Overflow post involves an entity relationship (ER) diagram representing a business table with a stars attribute. The goal is to categorize the values of this attribute into three categories: ‘2-3’, ‘4-5’, and ’none’. To achieve this, the developer attempts to use a SQL case statement as follows:

select 
  case b.stars
    when (b.stars >= 2.0 and b.stars <=3.0) then '2-3'
    when (b.stars >= 4.0) then '4-5'
    else 'none'
  end as stars_group
from business b

Why the Case Statement Won’t Work

The issue with this code is that it’s using a syntax similar to C or other procedural programming languages, but SQL doesn’t work in this way. In SQL, when and then are not keywords used to construct case statements.

The correct way to implement a case statement in SQL involves using the following syntax:

select 
  (case when condition1 then value1 else value2 end) as label
from table_name

In this revised code, we’re using a single case keyword to evaluate different conditions and return corresponding values.

Separate Conditions vs. Equality-Based Comparisons

There are two types of case statements in SQL: separate conditions and equality-based comparisons.

Separate Conditions

Separate conditions involve multiple when clauses followed by an else clause. Each when clause evaluates a condition, and if that condition is true, the corresponding value is returned. The else clause provides a default value to be used when none of the previous conditions are met.

select 
  (case 
       when b.stars >= 2.0 and b.stars <=3.0 then '2-3'
       when b.stars >= 4.0 then '4-5'
       else 'none'
     end) as stars_group
from business b

Equality-Based Comparisons

Equality-based comparisons, on the other hand, involve using equality operators (=, <, >, etc.) to compare values with specific conditions.

select 
  (case 
       when trunc(b.stars) = 2 then 'Two'
       when trunc(b.stars) = 3 then 'Three'
       else 'none'
     end) as stars_group
from business b

In this example, we’re using trunc to truncate the decimal part of the stars value and comparing it with specific values (2 and 3).

Searches vs. Simple Cases

When working with inequalities, SQL uses a concept called “searched cases.” This involves having multiple when clauses followed by no expression between case and where.

select 
  (case when b.stars >= 4.0 then '4-5' else null end) as stars_group
from business b

In this example, we’re using a searched case to evaluate the condition b.stars >= 4.0 and return the value '4-5' if true.

Conclusion

SQL case statements can be a powerful tool for making decisions based on specific conditions. However, understanding how to properly implement these statements is crucial for writing effective queries. In this article, we’ve explored the different types of SQL case statements, including separate conditions, equality-based comparisons, and searched cases. By mastering these concepts, developers can write more efficient and accurate queries that extract meaningful data from their databases.

Example Use Cases

• Categorizing employee ratings based on a scale of 1-5
• Assigning tax rates to customers based on their income levels
• Determining the number of days in a pay period based on a specific date range

Last modified on 2023-07-17