Database Management Systems
In PostgreSQL it’s possible to make an array larger without providing any values. For example, we can take an array with three elements, and enlarge it so that it contains say, five elements, with the two extra elements being NULL.
We can also do it so that some of the extra elements are non-NULL and others are NULL.
If you’re getting SQL Server error 1989 that reads something like “Cannot enable foreign key constraint ‘FK_Jobs_Employees’ as index ‘PK__Employee__7AD04FF1A39ECAB1’ on referenced key is disabled“, it’s probably because you’re trying to enable a foreign key, when the index that it references is disabled.
For example, if you’ve disabled a primary key on the parent table, and then try to enable a foreign key that references that primary key, then you’ll get this error.
To fix this issue, enable the index for the primary key/referenced key before enabling the foreign key.
Continue readingIndexes play an important role in SQL database performance. We can use them to speed up commonly run queries, so that users don’t have to sit and wait for their results to come in.
In SQL Server, indexes can be created in several different situations. For example, when we create a primary key or a UNIQUE constraint, an index is created behind the scenes for us. However, we can also create indexes explicitly using the CREATE INDEX statement.
When creating a foreign key constraint in SQL Server, we have the option of including ON DELETE SET NULL in the definition. When we use this option in a foreign key, it tells SQL Server to automatically set the foreign key column values to NULL in the child table when the corresponding primary key record in the parent table is deleted.
If you’re getting SQL Server error 2714 that reads something like “There is already an object named ‘Actors’ in the database” it seems that you’re trying to create an object that already exists.
It could be that you didn’t know that the object had already been created. Or it could be that you’re trying to create a different object, but you’re inadvertently using the same name that another object has. Or it could be that you want both objects to have the same name, but a different schema. In this case, perhaps you inadvertently omitted the schema name from the object.
Continue readingWhen using aggregation functions in PostgreSQL, we have the option of including a FILTER clause. This clause allows us to narrow down – or filter – the rows that are fed to the aggregate function.
This can be be a handy way to provide functionality that we might otherwise need to use a CASE statement or perhaps a CTE.
In this article, we’ll take a look at PostgreSQL’s FILTER clause, and see how it can simplify our SQL queries by replacing CASE statements with more concise code.
If you’re getting the “Incorrect syntax…” error in SQL Server when trying to create a filtered index, it could be that you’re using an unsupported operator.
Filtered indexes in SQL Server only support simple comparison operators. If you use an unsupported operator or expression, you’ll likely get the dreaded “Incorrect syntax” error.
To fix, either use a supported operator, or create a view.
Continue readingTransact-SQL (T-SQL) offers a nice set of functions and methods for manipulating strings. Let’s look at some handy techniques that can help us work more efficiently with string data in SQL Server.
Continue readingTemporal tables can be a powerful way to track historical data changes over time in our SQL Server databases. When we’re working with a database that uses temporal tables, we may need to identify all such tables quickly. Or we may simply want to check to see if it has any temporal tables.
Either way, the following SQL queries can be used to return a list of all temporal tables in the database.
Continue readingWhen attempting to truncate a table in PostgreSQL, you might encounter the error “cannot truncate a table referenced in a foreign key constraint“. This is the default behaviour for the TRUNCATE statement whenever you try to truncate a table that is referenced by another table through a foreign key relationship.
If you want to truncate both tables, you can use the CASCADE option of the TRUNCATE statement. Alternatively, you could truncate both tables at the same time.