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.
Continue readingIf 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.
Continue readingIf 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.
Continue readingDynamic 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.
Continue readingProbably 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.
Continue readingIf you’ve been working with prepared statements in PostgreSQL, you may get to a point where you no longer need one or more of them. In that case, you might want to remove them, or delete them, drop them, or however you want to say it.
Fortunately, this is one of the quickest, easiest things to do in PostgreSQL.
Continue readingA common table expression (CTE) in SQL Server is used to create a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement. A CTE can simplify complex queries and improve readability. However, there are specific rules for using the ORDER BY clause inside a CTE.
If you’re getting an error that reads something like “Column names in each table must be unique. Column name ‘c1’ in table ‘t1’ is specified more than once” in SQL Server, it appears that you’re trying to create a table with duplicate column names, or you’re trying to add a column to a table that already has a column of that name.
Column names must be unique within each table.
Continue readingError 156 in SQL Server is a generic error that doesn’t tell us much, other than the fact that we’re using the wrong syntax.
But if you’re running a subquery, and you’re getting error 156, which reads “Incorrect syntax near the keyword ‘WHERE’.” or whatever keyword it’s near, it could be that you haven’t declared an alias for the subquery.
Continue readingIn MySQL, a UNIQUE constraint is a constraint type that ensures that all values in a column or a group of columns are distinct from each other. In other words, all values that go into the column or group of columns must be unique.
UNIQUE constraints can be applied whenever we want to prevent duplicate entries in specific columns without making them a primary key.