Complete Guide to SQL Server Data Types

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SQL Server provides a solid set of system data types that handle everything from storing tiny integers to massive text blobs. Understanding these types is an important part of designing efficient databases, mainly because picking the right data type can save storage space and improve query performance.

This article breaks down all the data types available in SQL Server (as of SQL Server 2025), organized by category. Each type includes its max length, precision, scale, and whether it can be nullable.

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Fix Error 4901 “ALTER TABLE only allows columns to be added that can contain nulls… etc” in SQL Server

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If you’re getting an error in SQL Server that reads something like “ALTER TABLE only allows columns to be added that can contain nulls, or have a DEFAULT definition specified, or the column being added is an identity or timestamp column, or… etc etc“, it’s probably because you’re trying to add a NOT NULL column to a table that already contains data.

This error occurs due to the fact that the NOT NULL constraint will be violated for every row in the table. Think about it for a second. When you first add the column, there’s no data. It’s not until you run a subsequent INSERT statement (or some other process that populates the table) that you will get data. In the meantime, all values in your new column will be NULL. And that, of course, violates the NOT NULL constraint.

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What is Relationship Cardinality?

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When you’re designing a relational database, you need to define how tables relate to each other. You do this by creating relationships between the tables. Relationship cardinality describes how many records in one table can be associated with records in another table. It’s a fundamental concept that affects how you structure your database and write your queries.

Understanding cardinality helps you avoid design mistakes that lead to data integrity issues, performance problems, or queries that return unexpected results.

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What is a Vector Database?

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A vector database is a specialized database designed to store, index, and query high-dimensional vectors. These are arrays of numbers that represent data in mathematical space. Unlike traditional databases that store text, numbers, or structured data, vector databases work with embeddings, which are numerical representations of complex data like text, images, audio, or video that capture their semantic meaning.

These databases solve the specific problem of finding similar items based on meaning rather than exact matches. Traditional databases excel at finding exact matches or simple comparisons. For example “find all users named Bella” or “find products under $50.” Vector databases excel at similarity searches. These could look something like “find images similar to this one” or “find documents with similar meaning to this query,” even when the exact words or pixels are different.

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Using Subqueries with SQL Server’s DATEDIFF() Function

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While SQL Server’s DATEDIFF() function is relatively straightforward when you’re comparing two known dates, it becomes more flexible when the dates you compare are sourced directly from your tables. Instead of hardcoding dates, you can embed subqueries directly into the DATEDIFF() function to dynamically retrieve the dates you need.

This approach can be especially useful when you’re working with aggregate functions like MIN() and MAX(), or when you need to pull specific dates based on certain conditions. The subqueries execute first, return their date values, and DATEDIFF() uses those results to perform the calculation.

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PIVOT vs GROUP BY in SQL Server

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If you’re new to SQL or just starting to explore data transformation techniques, you might be confused about when to use PIVOT versus GROUP BY. They may seem similar at first. After all, both aggregate data, both can summarize information, and both produce condensed result sets. But they serve fundamentally different purposes and produce very different output structures.

Understanding the distinction between these two operations will help you write more effective queries. GROUP BY aggregates data vertically, keeping your results in a row-based format. PIVOT transforms data horizontally, turning row values into column headers. Let’s break down exactly what each does and when you’d use one over the other.

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SQL Server SUBSTRING(): A Complete Guide to Extracting Strings

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The SUBSTRING() function in SQL Server lets you pull out specific portions of a string. Whether you’re cleaning data, formatting output, or parsing complex text fields, this function is one you’ll use constantly.

If you’ve ever needed to extract an area code from a phone number, grab the domain from an email address, or parse product codes into their component parts, SUBSTRING() is your go-to tool. It’s pretty straightforward, and once you understand how it works, you’ll find yourself using it all the time.

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