Column Constraints vs Table Constraints in SQL: What’s the Difference?

Posted on by Ian

In relational database management systems (RDBMSs), constraints are nifty tools that we can use to ensure the integrity, accuracy, and reliability of the data stored in our database.

Constraints can enforce rules at the column and table levels, guiding how data can be inserted, updated, or deleted. Whether you’re defining the uniqueness of a value, establishing relationships between tables, or ensuring that critical fields are never left blank, constraints play an important role in the design of relational databases.

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Understanding the SERIAL Type in MySQL

Posted on by Ian

If you work with MySQL databases, you may have encountered the SERIAL type in a table’s definition. And if you’ve come over from PostgreSQL, you might have a false expectation about how MySQL’s SERIAL works – unless you already know 😉

Below is a quick overview of MySQL’s SERIAL type, including an explanation of how it differs from PostgreSQL’s SERIAL type and similar functionality from other DBMSs.

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How to Disable a Constraint in SQL Server

Posted on by Ian

Disabling constraints in SQL Server can be useful for various operations, such as bulk data loading or certain maintenance tasks. But not all constraints are created equal. The method we use to disable a constraint, and whether that’s even possible, depends on the type of constraint.

In this article, we’ll explore how to disable different types of constraints and consider the potential impacts.

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Fixing Error 4121 When Using a CTE with FOR XML in SQL Server

Posted on by Ian

If you’re trying to use a common table expression (CTE) that produces XML in SQL Server, but you’re getting error 4121, which reads something like “Cannot find either column “EmployeeData” or the user-defined function or aggregate “EmployeeData.query”, or the name is ambiguous” it might be that you’re trying to query the XML in the outer query, but your CTE isn’t outputting the XML as an actual xml type.

The FOR XML result must be an xml type in order to process the result on the server.

To fix this issue, make sure the CTE outputs the result using the xml data type.

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Fix “Unknown table … in HANDLER” in MySQL (Error 1109)

Posted on by Ian

If you’re getting MySQL error 1109 which reads something like “1109 (42S02): Unknown table ‘products’ in HANDLER” in MySQL, it appears that you’re trying to reference a table that’s not currently open when using MySQL’s HANDLER statement.

This can happen when you try to read from a table that you haven’t yet opened. It can also happen when you’ve assigned an alias to the table but you try to reference it without the alias. And it can happen if you try to close a table that isn’t actually open.

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4 SQL Server Dynamic Management Views That Return Memory Usage Data

Posted on by Ian

Dynamic Management Views (DMVs) in SQL Server can be quite handy for monitoring and troubleshooting database performance. When it comes to memory management, understanding how our SQL Server instance uses memory can help us identity performance issues, as well as potential fixes.

In this article, we’ll explore four essential DMVs that provide valuable insights into memory usage in SQL Server.

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Understanding the Different Types of Keys in SQL

Posted on by Ian

Probably the most widely known key type in SQL is the primary key, which is chosen to uniquely identify each row in a table. Perhaps next is the foreign key, which is used to establish a relationship between tables.

But there are more key types than this, and the differences between them can be subtle, but important. Here we’ll look at nine of the various key types in SQL.

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