If you’re getting SQL Server error 107 that goes something like “The column prefix ‘e’ does not match with a table name or alias name used in the query“, look at your column prefix.
This error occurs when we use the asterisk wildcard (*) that’s incorrectly qualified with a column prefix. The prefix, if provided, must match a table name or alias name. If you’re using a table alias, then you must use that instead of the table name.
When writing queries in SQL, we sometimes need to summarize data based on specific criteria. Fortunately SQL makes this easy to achieve with the GROUP BY and HAVING clauses. These two clauses allow us to aggregate data and filter the results based on group conditions.
More specifically, GROUP BY allows us to group the results, and HAVING is an optional clause that we can use to filter the results based on certain conditions.
Below is an example of where we can use these two clauses to aggregate data and filter it based on certain criteria.
Continue readingIn relational database design, the concept of a “key” is fundamental. Keys are essential for uniquely identifying records in a table, ensuring data integrity, and facilitating efficient data retrieval. Among the different types of keys used in SQL, the natural key is one that often sparks discussion among database designers, especially when compared with the surrogate key.
Let’s explore the concept of a natural key, its advantages and disadvantages, and look at a practical example to illustrate its use.
Continue readingWhile the primary key is almost certainly the most discussed key in SQL, there are other types of keys that we shouldn’t forget. One such key is the alternate key.
In this article, we’ll look at the concept of the alternate key in SQL.
Continue readingThere are many different “key” types in SQL. We have the primary key, the foreign key, and even the unique key.
But have you heard about the candidate key?
Continue readingYou may be aware that PostgreSQL has a couple of functions that allow us to concatenate strings. In particular, the concat() function allows us to concatenate multiple strings into one string, and the concat_ws() function allows us to do the same, but to also specify a separator for the concatenated strings.
But did you know that we have the ability to pass an array to these functions?
Continue readingIf you’re getting SQL Server error 547 that reads something like “The DELETE statement conflicted with the REFERENCE constraint “FK_Projects_DepartmentID”. The conflict occurred in database “test”, table “dbo.Projects”, column ‘DepartmentID’“, you’re probably trying to delete data from a parent table of a foreign key relationship.
Continue readingOne of the core principles that ensure data integrity in relational database management systems (RDBMSs) is atomicity. When dealing with SQL, understanding what “atomicity” and “atomic” means can help you design more reliable and robust databases.
Atomicity is a key concept that underpins how SQL transactions operate, ensuring that either all steps in a transaction are completed successfully, or none are. This article looks at the meaning of atomic in SQL, its importance, and how it fits into the broader context of database management.
Continue readingIn SQL Server, foreign keys are essential for maintaining referential integrity between tables. When setting up a foreign key, we have various options for handling actions when the referenced data is deleted. One of these options is ON DELETE SET DEFAULT.
This feature can be particularly useful in scenarios where it’s important to maintain the relationship while ensuring that the foreign key column doesn’t end up with invalid or null values.
Continue readingIf you’re getting SQL Server error 1785 that reads something like “Introducing FOREIGN KEY constraint ‘FK__Name’ on table ‘Department’ may cause cycles or multiple cascade paths. Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints“, it’s probably because you’ve specified the CASCADE option when trying to create a foreign key with a circular reference, or one that has multiple cascade paths (perhaps your child table has multiple foreign keys to the same primary key on the parent table).