Tag: constraints

Why SQLite Allows NULL Values in Primary Key Columns

Posted on by Ian

SQLite, one of the most widely used database engines, is known for its lightweight design, ease of use, and adherence to most aspects of the SQL standard. However, one notable deviation from the standard lies in its handling of PRIMARY KEY constraints. Unlike the SQL standard, SQLite allows NULL values in primary key columns in some cases.

Let’s look at the reasons behind this behavior, and explore the implications of NULL values in primary key columns. We’ll also examine SQLite’s treatment of NULL values as distinct for uniqueness constraints.

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Why the Primary Key Might Not Appear in PRAGMA index_list() in SQLite

Posted on by Ian

In most relational database management systems (RDBMSs) the PRIMARY KEY is used to define the unique row identifier for a table. But in SQLite, not all primary keys are handled the same way when it comes to indexing.

Depending on how the primary key is defined in a table, it may or may not show up in the list of indexes returned by the PRAGMA index_list() command. In particular, when the primary key is an INTEGER PRIMARY KEY, SQLite doesn’t explicitly create a separate index for it.

This article will explain why this happens and provide examples with different types of primary key definitions.

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Create a DEFAULT Constraint in MySQL

Posted on by Ian

In MySQL, a DEFAULT constraint is used to provide a default value for a column when no value is specified during an INSERT operation. This is particularly useful for ensuring that important columns have consistent, non-null values, even when omitted from inserts.

In this article, we’ll use MySQL to create a table with a couple of DEFAULT constraints, and we’ll also add a constraint to that table after it has been created.

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Fix “Violation of PRIMARY KEY constraint” in SQL Server (Error 2627)

Posted on by Ian

If you’re getting an error that reads something like “Violation of PRIMARY KEY constraint ‘PK_CatId’. Cannot insert duplicate key in object ‘dbo.Cats’. The duplicate key value is (1)” in SQL Server, it’s because you’re trying to insert a duplicate value into a primary key column.

A primary key cannot contain duplicate values.

To fix this issue, you’ll need to change the value you’re trying to insert into the primary key column.

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Why it’s a Good Idea to Create Indexes on Foreign Keys in PostgreSQL

Posted on by Ian

When a foreign key is defined in a table, it ensures that the values in the column(s) correspond to values in a primary key or unique key in another table. While PostgreSQL automatically creates an index for primary keys (because these need to be fast for lookups and enforcing uniqueness), it does not automatically create an index for foreign keys.

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Identify a Table’s Primary Key’s Index in SQL Server

Posted on by Ian

There are many ways to find the primary key column in SQL Server, but sometimes we might need to identify the name of the primary key’s index.

For example, we may want to identify the index’s name if we need to disable the primary key for some reason (we disable primary keys by disabling their index).

The query below can help if you need to find the name of a primary key’s index for a given table.

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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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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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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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How to Create a UNIQUE Constraint in MySQL

Posted on by Ian

In 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.

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