How to use SQL SET/DECLARE Statements (with example)

In this post, we’ll explore how to declare and assign variables using the SET and DECLARE statements in SQL. Utilizing variables in SQL is crucial for efficient data management and analysis tasks. We’ll understand the correct usage of these statements, learn through actual examples, and examine considerations when using variables.

Example: Monthly Sales Analysis

As a data analyst, you are tasked with analyzing your company’s monthly sales performance and reporting to the management team. The sales data is stored in the Sales table, which includes the date of each sale (SaleDate), the sales amount (SalesAmount), and the customer ID (CustomerID). The management team is particularly interested in the total sales amount and the number of transactions for the past month. We will use the DECLARE and SET statements to dynamically calculate the start and end dates of the last month and query the total sales amount and number of transactions for that period based on SQL (the SQL query example is provided below).

Overview of SET and DECLARE Statements

In SQL, variables act as temporary storage for data. The DECLARE statement is used for variable declaration, and the SET statement is used to assign values to the declared variables.

DECLARE: Declares a variable and specifies its data type. The declared variable can then be assigned values and used within the script.
SET: Assigns a specific value to a declared variable. The value can be a fixed value, a calculated result, or the outcome of a query.

Declaring Variables (Using the DECLARE Statement)

When declaring variables, you prefix the variable name with the @ symbol and specify its data type. For example, to declare a date type variable:

DECLARE @StartDate DATE;

This code declares a date type variable named @StartDate.

Assigning Values to Variables (Using the SET Statement/Assigning Dynamic Values)

The SET statement is used to assign values to declared variables. The value can be directly specified or dynamically assigned from other variables, functions’ results, etc.

SET @StartDate = '2024-01-01';

Or, an example of dynamically calculating and assigning a date would be:

Learning Through Examples

Let’s revisit the initial example. We want to dynamically calculate the first and last days of the last month and use these dates in our monthly query.

The example query follows these steps:

Variable Declaration: Declare the @FirstDayOfLastMonth and @LastDayOfLastMonth variables as DATE types.
Assigning Values to Variables: Use the DATEADD and DATEDIFF functions to calculate the first and last days of the last month, and assign these values to the variables.
Querying Sales Data: Use the variables in the WHERE clause to calculate the total sales amount and the number of transactions for the last month.

-- Declaring variables to calculate the first and last day of the month
DECLARE @FirstDayOfLastMonth DATE, @LastDayOfLastMonth DATE;

-- Calculating the first day of the last month
SET @FirstDayOfLastMonth = DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()) - 1, 0);

-- Calculating the last day of the last month
SET @LastDayOfLastMonth = DATEADD(DAY, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()), 0));

-- Get the sales total and number of transactions for the last month
SELECT
    SUM(SalesAmount) AS TotalSales,
    COUNT(*) AS NumberOfTransactions
FROM
    Sales
WHERE
    SaleDate >= @FirstDayOfLastMonth AND
    SaleDate <= @LastDayOfLastMonth;

This example demonstrates how SQL variables can be used to calculate dynamic date ranges and base data queries on them. By leveraging this approach in business, repetitive data analysis tasks can be automated each month, providing more accurate and efficient information.

Considerations When Using Variables

Understanding Scope: Variables are only valid within the batch or procedure where they are declared. They cannot be accessed in other batches or procedures.
Performance Optimization Tips: When using variables to execute queries repeatedly, enhancing the reuse potential of the query can optimize performance.
Advantages and Disadvantages of Using Variables: Variables improve code readability and facilitate the writing of dynamic queries. However, excessive use of variables may lead to increased memory usage, so caution is advised.