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Mastering SQL Server CASE Expression: A Complete Guide

SQL Server CASE Expression in Queries

In the world of SQL, the CASE expression is a powerful tool for manipulating data within queries. This article will discuss two types of CASE expressions: simple and searched.

We will also provide examples of how to use these expressions in SELECT statements and aggregate functions.

Simple CASE Expression

A simple CASE expression provides a way to evaluate a single expression and perform a specific action based on that evaluation. The basic syntax is:

“`

CASE expression

WHEN value1 THEN result1

WHEN value2 THEN result2

ELSE result3

END

“`

We use the CASE keyword followed by the expression we want to evaluate. This can be any valid expression, such as a column name or a mathematical formula.

We then specify the different possible values that the expression can have and the corresponding results. Finally, we use the ELSE keyword to specify a result if none of the WHEN clauses match.

Simple CASE Expression in SELECT Clause Example

Let’s see how we can use a simple CASE expression in a SELECT statement. Consider the following example:

“`

SELECT CASE grade

WHEN ‘A’ THEN ‘Excellent’

WHEN ‘B’ THEN ‘Good’

WHEN ‘C’ THEN ‘Average’

ELSE ‘Below Average’

END as GradeLabel,

COUNT(*) as TotalStudents

FROM Students

GROUP BY grade;

“`

In this query, we want to group students by their grade and count the number of students in each group. We also want to add a label to each grade based on their letter grade, such as ‘Excellent’ for an ‘A’, ‘Good’ for a ‘B’, and so on.

The CASE expression allows us to do this easily. We use the CASE keyword followed by the ‘grade’ column name and specify the different values and labels we want to use.

We then include the result of the CASE expression in the SELECT clause using the alias ‘GradeLabel’.

Simple CASE Expression in Aggregate Function Example

Next, let’s look at an example of using a simple CASE expression in an aggregate function. Consider this query:

“`

SELECT SUM(CASE

WHEN gender = ‘M’ THEN 1

ELSE 0

END) as TotalMen,

COUNT(*) as TotalStudents

FROM Students;

“`

In this example, we want to count the total number of male students in our table. We use the SUM function and a simple CASE expression to achieve this.

We set the expression to 1 when the gender is ‘M’ and 0 otherwise. The SUM function then counts the number of 1’s to give us the total number of male students.

We also include the total number of students in the table using the COUNT function.

Searched CASE Expression

A searched CASE expression provides a way to evaluate multiple conditions and perform a specific action based on each condition. The basic syntax is:

“`

CASE

WHEN condition1 THEN result1

WHEN condition2 THEN result2

ELSE result3

END

“`

We use the CASE keyword followed by a list of multiple WHEN clauses, each with a condition to evaluate. The conditions can include operators like greater than or less than or any other valid expression.

We then specify the corresponding result for each condition, followed by an ELSE clause to specify a result if none of the conditions match.

Searched CASE Expression in SELECT Clause Example

Let’s see how we can use a searched CASE expression in a SELECT statement. Consider the following example:

“`

SELECT Students.StudentID,

Students.FirstName,

Students.LastName,

Departments.DepartmentName,

CASE

WHEN Students.TotalCredits >= 60 AND Departments.DepartmentID = 1 THEN ‘Graduating’

WHEN Students.TotalCredits >= 90 AND Departments.DepartmentID = 2 THEN ‘Graduating’

ELSE ”

END as GraduationStatus

FROM Students

INNER JOIN Departments ON Students.DepartmentID = Departments.DepartmentID;

“`

In this query, we want to join two tables (Students and Departments) and retrieve information about each student, including their graduation status. We use a searched CASE expression to evaluate the student’s total credits and department ID and assign a graduation status based on these conditions.

Syntax of SQL Server CASE Expression

It’s important to know the syntax of the various CASE expressions in SQL Server so you can use them effectively in your queries.

Simple CASE Expression Syntax

The simple CASE expression syntax is:

“`

CASE expression

WHEN value1 THEN result1

WHEN value2 THEN result2

ELSE result3

END

“`

We use the CASE keyword followed by the expression to evaluate. The expression can be any valid expression, such as a column name or a mathematical formula.

We then specify the different possible values and the corresponding results.

Searched CASE Expression Syntax

The searched CASE expression syntax is:

“`

CASE

WHEN condition1 THEN result1

WHEN condition2 THEN result2

ELSE result3

END

“`

We use the CASE keyword followed by a list of multiple WHEN clauses, each with a condition to evaluate. We then specify the corresponding result for each condition, followed by an ELSE clause to specify a result if none of the conditions match.

Conclusion

In summary, the SQL Server CASE expression is a powerful tool for manipulating data within queries. We’ve covered two types of CASE expressions in this article: simple and searched.

We’ve also provided examples of how to use these expressions in SELECT statements and aggregate functions. By understanding the syntax of the CASE expression, you can use it effectively in your own queries.

Working of SQL Server CASE Expression

The SQL Server CASE expression is a flexible tool that allows you to manipulate and transform data within queries. There are two types of SQL Server CASE expressions: simple and searched.

These expressions work in a similar way but with some important differences. Evaluation of

Simple CASE Expression

The evaluation of a simple CASE expression is straightforward.

When the expression is evaluated, it returns a single result based on the first condition that is true. If none of the conditions are true, then the ELSE clause is executed.

Here’s an example:

“`

SELECT CASE grade

WHEN ‘A’ THEN ‘Excellent’

WHEN ‘B’ THEN ‘Good’

WHEN ‘C’ THEN ‘Average’

ELSE ‘Below Average’

END as GradeLabel

FROM Students;

“`

In this example, we use a simple CASE expression to assign label grades to students based on their letter grades. If a student has an ‘A’, the result is ‘Excellent’.

If a student has a ‘B’, the result is ‘Good’. If a student has a ‘C’, the result is ‘Average’.

If a student has any other letter grade, the result is ‘Below Average’. Evaluation of

Searched CASE Expression

A searched CASE expression, on the other hand, evaluates multiple conditions, one by one, until a condition is met.

Once a condition is met, it returns the corresponding result. If none of the conditions are met, then the ELSE clause is executed.

Here’s an example:

“`

SELECT CASE

WHEN age < 18 THEN 'Minor'

WHEN age >= 18 AND age < 65 THEN 'Adult'

WHEN age >= 65 THEN ‘Senior’

ELSE ‘Unknown’

END as AgeGroup

FROM Customers;

“`

In this example, we use a searched CASE expression to assign age groups to customers based on their ages. Customers who are under 18 years old are considered ‘Minors’, while customers who are between 18 and 64 years old are considered ‘Adults’.

Customers who are 65 or older are classified as ‘Seniors’. If a customer’s age does not fall into any of these categories, then the result is ‘Unknown’.

Optional ELSE Statement

The ELSE statement is optional in both simple and searched CASE expressions. However, leaving out the ELSE clause can cause issues if none of the conditions are met.

If this happens, the result will be NULL. So, it’s always a good idea to include an ELSE statement to cover all possible scenarios.

Examples of Using SQL Server CASE Expression

Let’s take a look at some examples of using SQL Server CASE expressions in queries. Example of

Simple CASE Expression in SELECT Clause

“`

SELECT FirstName, LastName,

CASE gender

WHEN ‘M’ THEN ‘Male’

WHEN ‘F’ THEN ‘Female’

ELSE ‘Unknown’

END as Gender

FROM Customers;

“`

This simple SELECT statement retrieves customer names and genders from the Customers table.

Instead of returning the original gender values, we use a simple CASE expression to replace them with clear ‘Male’, ‘Female’, or ‘Unknown’ strings. Example of

Simple CASE Expression in Aggregate Function

“`

SELECT SUM(CASE

WHEN gender = ‘M’ THEN 1

ELSE 0

END) as TotalMen,

SUM(CASE

WHEN gender = ‘F’ THEN 1

ELSE 0

END) as TotalWomen

FROM Customers;

“`

This query uses two separate COUNT functions to determine the total number of male and female customers. We use a simple CASE expression with the SUM function to count all instances of customers classified as ‘M’ or ‘F’.

Example of

Searched CASE Expression in SELECT Clause

“`

SELECT FirstName, LastName, City,

CASE

WHEN City = ‘New York’ THEN ‘East Coast’

WHEN City = ‘Los Angeles’ THEN ‘West Coast’

WHEN City = ‘Chicago’ THEN ‘Midwest’

ELSE ‘Unknown’

END as Region

FROM Customers;

“`

In this example, we retrieve customer information from the Customers table along with their regions based on their cities. A searched CASE expression evaluates the customer city and assigns a region based on the corresponding condition.

Customers from New York are in the ‘East Coast’ region, those from Los Angeles are in the ‘West Coast’ region, those from Chicago are in the ‘Midwest’ region, and all other customers are classified as ‘Unknown’.

In conclusion, SQL Server CASE expressions provide a powerful and flexible way to manipulate and transform data within queries.

Simple CASE expressions allow you to replace values based on a single condition, while searched CASE expressions evaluate multiple conditions. Including an ELSE statement in your CASE expressions is always a good idea to cover all possible scenarios.

By using these expressions effectively in your SQL Server queries, you can enhance data analysis and reporting capabilities. In conclusion, the SQL Server CASE expression is an essential tool for manipulating and transforming data in SQL queries.

This article explored two types of CASE expressions, simple and searched, along with their syntax, evaluation, and examples of use in SELECT statements and aggregate functions. By applying these CASE expressions effectively, you can enhance your data analysis and reporting capabilities, making decision-making and problem-solving more effective.

When using CASE expressions, including an ELSE statement is a good practice to cover all possible scenarios. In summary, SQL Server CASE expression is a powerful and flexible tool that can make a significant difference in data management and analysis.

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