inding data in a relational database is usually a simple task, especially if you’re simply comparing entries to a specific value, but queries rapidly become more complicated when you start hunting for data that’s missing, in other words, data that could exist, but doesn’t. Missing data is data related to existing data that simply doesn’t occur. You could find missing values using a two-query solution, but SQL’s EXISTS predicate can help you quickly find missing values with just one query?a subquery to be exact.
For example, you may create a query to find active customers by counting recent orders. However, finding inactive customers may be just as important; perhaps you can learn why they’re inactive and perhaps turn them into active customers. In this article, you’ll see how to use the SQL EXISTS predicate to find missing data. This article is aimed at the Jet and Transact-SQL (T-SQL) audience. We’ll offer specific instructions when syntax or rules differ between the two dialects.
|This article refers to Jet and Transact-SQL. The examples work in Access 97, 2000, and 2002. The T-SQL syntax is correct for SQL Server 2000.|
What Is EXISTS?
SQL’s EXISTS predicate specifies a subquery and then compares a value against the existence of one or more rows in that subquery. The subquery returns True when the subquery contains any rows and False when it doesn’t. However, all of that is probably clear as mud if you’re not familiar with subqueries.
A subquery is a SELECT query within another SELECT, INSERT, UPDATE, DELETE query, or another subquery. In other words, the subquery returns a result set, which is then subject to the main query’s conditions and criteria. Think of the subquery as a filter. The results of the embedded SELECT, also known as the inner query or inner select, become part of the search condition for the main query, otherwise known as the outer query or outer select.
The subquery can take many forms:
SELECT field1?, (subquery) AS alias FROM datasource SELECT fieldlist FROM datasource WHERE field comparison operator (subquery) SELECT fieldlist FROM datasource WHERE field ANY|SOME|ALL (subquery) WHERE expression [NOT] EXISTS (subquery)
In the preceding code, comparison operator equals one of the following: =, <>, <, >, >=, <, !>, !lt;, or <=. When you use one of these comparison operators, the subquery must return a single value. In this article, you'll work with the last form?the one that includes the EXISTS predicate.
Create a Subquery with EXISTS
When combined with EXISTS, an outer query checks for the existence of values within the inner query’s result set. Now, using the Northwind sample database that comes with Access, suppose you want to learn which companies have placed orders. The solution is simple?you don’t need a complex EXISTS solution just yet. The following query returns a unique list of companies with records in the Orders table (see Figure 1):
SELECT DISTINCT Orders.CustomerID FROM Orders
There are 89 companies that have placed at least one order (see Figure 1). The Access query displays the company name instead of the CustomerID value because the CustomerID field in the Orders table is a lookup field. A lookup field is an Access data type that displays a value other than the value that’s actually stored (SQL Server 2000 also supports lookup fields via an ADP front-end).
The inner query compares the CustomerID value for each record in the Orders table to the CustomerID values in the Customers table. When a matching value is found, the inner query returns True. A True value means the customer has placed an order, but the NOT operator preceding the EXISTS predicate eliminates that particular value from the outer query’s result set.
When the inner query doesn’t find any matching records, it returns False. Remember, a False value means the customer hasn’t ordered. The NOT operator then negates that False value, so the outer query can include that record in its result set. The EXISTS predicate finds values that do exist; preceding that predicate with the NOT operator finds those that don’t.
Missing data can often be just as informative as data that’s available. Unfortunately, learning what’s missing isn’t always as easy as finding existing data. Combining SQL’s EXISTS predicate with the NOT operator solves the problem and quickly turns up missing information.
Share the Post:
GM Creates Open Source uProtocol and Invites Automakers to Adopt It: Revolutionizing Automotive Software Development.
General Motors (GM) recently announced its entry into the Eclipse Foundation. The Eclipse Foundation is a prominent open-source software foundation. In addition, GMC announced its contribution of “uProtocol” to facilitate
What is Metadata?
What is metadata? Well, It’s an odd concept to wrap your head around. Metadata is essentially the secondary layer of data that tracks details about the “regular” data. The regular
What We Should Expect from Cell Phone Tech in the Near Future
The earliest cell phones included boxy designs full of buttons and antennas, and they only made calls. Needless to say, we’ve come a long way from those classic brick phones
The Best Mechanical Keyboards For Programmers: Where To Find Them
When it comes to programming, a good mechanical keyboard can make all the difference. Naturally, you would want one of the best mechanical keyboards for programmers. But with so many
The Digital Panopticon: Is Big Brother Always Watching Us Online?
In the age of digital transformation, the internet has become a ubiquitous part of our lives. From socializing, shopping, and learning to more sensitive activities such as banking and healthcare,
Embracing Change: How AI Is Revolutionizing the Developer’s Role
The world of software development is changing drastically with the introduction of Artificial Intelligence and Machine Learning technologies. In the past, software developers were in charge of the entire development
The Benefits of Using XDR Solutions
Cybercriminals constantly adapt their strategies, developing newer, more powerful, and intelligent ways to attack your network. Since security professionals must innovate as well, more conventional endpoint detection solutions have evolved
How AI is Revolutionizing Fraud Detection
Artificial intelligence – commonly known as AI – means a form of technology with multiple uses. As a result, it has become extremely valuable to a number of businesses across
Companies Leading AI Innovation in 2023
Artificial intelligence (AI) has been transforming industries and revolutionizing business operations. AI’s potential to enhance efficiency and productivity has become crucial to many businesses. As we move into 2023, several
Step-by-Step Guide to Properly Copyright Your Website
Creating a website is not easy, but protecting your website is equally important. Implementing copyright laws ensures that the substance of your website remains secure and sheltered. Copyrighting your website
Fivetran Pricing Explained
One of the biggest trends of the 21st century is the massive surge in analytics. Analytics is the process of utilizing data to drive future decision-making. With so much of
Kubernetes Logging: What You Need to Know
Kubernetes from Google is one of the most popular open-source and free container management solutions made to make managing and deploying applications easier. It has a solid architecture that makes