Understanding Case in SQL Server

Hey Dev, are you looking for more information on case statements in SQL Server? Look no further! In this journal article, we’ll dive into the basics of case statements, how they work, and how you can use them in your queries.

What is a Case Statement?

A case statement in SQL Server is a conditional statement that allows you to control the flow of your query based on certain conditions. It’s similar to an if/else statement in programming, but it’s used specifically for data manipulation in SQL. You can use case statements to create new columns, group data, or perform calculations.

Syntax

The basic syntax for a case statement is as follows:

Keyword	Description
case	Start of the case statement
when	A condition to check
then	The result of the condition if it’s true
else	The result of the condition if it’s false (optional)
end	End of the case statement

Example

Let’s see an example of how to use a case statement:

SELECTCASEWHEN Score >= 90 THEN 'A'WHEN Score >= 80 THEN 'B'WHEN Score >= 70 THEN 'C'ELSE 'F'END AS GradeFROM Students

In this example, we’re selecting the grade based on the student’s score. If their score is greater than or equal to 90, they get an A. If their score is between 80 and 90, they get a B, and so on. If their score is below 70, they get an F.

Using Case Statements in SQL Server

Now that we’ve covered the basics of case statements, let’s look at some practical examples of how to use them in SQL Server.

Creating New Columns

One of the most common uses of case statements is to create new columns in your query results. For example, let’s say you have a table of orders and you want to create a column that shows whether the order was shipped or not:

SELECTOrderID,OrderDate,CASEWHEN ShippedDate IS NULL THEN 'Not Shipped'ELSE 'Shipped'END AS StatusFROM Orders

In this example, we’re checking whether the ShippedDate column is null or not. If it’s null, we display “Not Shipped” in the Status column. If it’s not null, we display “Shipped”.

Grouping Data

You can also use case statements to group data in your query. Let’s say you have a table of employees and you want to group them by their salary range:

SELECTCASEWHEN Salary <= 50000 THEN 'Low'WHEN Salary <= 80000 THEN 'Medium'ELSE 'High'END AS SalaryRange,COUNT(*) AS EmployeeCountFROM EmployeesGROUP BYCASEWHEN Salary <= 50000 THEN 'Low'WHEN Salary <= 80000 THEN 'Medium'ELSE 'High'END

In this example, we’re grouping employees based on their salary range. If their salary is less than or equal to 50,000, they’re considered “Low”. If their salary is between 50,000 and 80,000, they’re considered “Medium”. If their salary is greater than 80,000, they’re considered “High”.

Performing Calculations

You can also use case statements to perform calculations in your query. Let’s say you have a table of products and you want to calculate their total cost based on their quantity:

SELECTProductID,Quantity,UnitCost,CASEWHEN Quantity >= 100 THEN Quantity * UnitCost * 0.9ELSE Quantity * UnitCostEND AS TotalCostFROM Products

In this example, we’re checking whether the quantity is greater than or equal to 100. If it is, we apply a 10% discount to the total cost. If it’s not, we just calculate the total cost normally.

FAQs

What is the difference between a case statement and an if/else statement?

In SQL Server, case statements are used specifically for data manipulation, while if/else statements are used for programming logic. Case statements are often used to create new columns, group data, or perform calculations, while if/else statements are used to control program flow.

Can you use multiple conditions in a case statement?

Yes, you can use multiple conditions in a case statement by using nested when statements. For example:

SELECTCASEWHEN Score >= 90 THEN 'A'WHEN Score >= 80 AND Score < 90 THEN 'B'WHEN Score >= 70 AND Score < 80 THEN 'C'ELSE 'F'END AS GradeFROM Students

In this example, we’ve added additional conditions to the B and C grades to further refine the results.

Can you use a case statement in a where clause?

Yes, you can use a case statement in a where clause to filter your results based on certain conditions. For example:

SELECTOrderID,OrderDate,CASEWHEN ShippedDate IS NULL THEN 'Not Shipped'ELSE 'Shipped'END AS StatusFROM OrdersWHERECASEWHEN ShippedDate IS NULL THEN 'Not Shipped'ELSE 'Shipped'END = 'Shipped'

In this example, we’re only returning orders that have been shipped.

Can you use a case statement in an order by clause?

Yes, you can use a case statement in an order by clause to sort your results based on certain conditions. For example:

SELECTProductName,UnitPrice,CASEWHEN UnitsInStock <= 10 THEN 'Low'WHEN UnitsInStock <= 50 THEN 'Medium'ELSE 'High'END AS StockLevelFROM ProductsORDER BYCASEWHEN UnitsInStock <= 10 THEN 'Low'WHEN UnitsInStock <= 50 THEN 'Medium'ELSE 'High'END DESC,UnitPrice ASC

In this example, we’re sorting products first by stock level in descending order (i.e. Low comes before Medium, which comes before High), and then by unit price in ascending order.