Draft

7.4 Assertions

Assertions

  • Introduction to assertions as a powerful form of active elements in SQL, which are part of the database schema.
  • Definition of an assertion as a boolean-valued SQL expression that must be true at all times.
  • Comparison between assertions and triggers, noting that assertions are easier for programmers to state but harder for DBMS to implement efficiently.
#1

read: Section 7.4: Assertions (section)

model: pro
rules: bullet definition math presentation short


Present the following.  Use definition formal definition when it applies.

- Introduction to assertions as a powerful form of active elements in SQL, which are part of the database schema.
- Definition of an assertion as a boolean-valued SQL expression that must be true at all times.
- Comparison between assertions and triggers, noting that assertions are easier for programmers to state but harder for DBMS to implement efficiently.

Assertions and triggers represent the most powerful forms of active elements in SQL. Unlike constraints tied to specific tuples or attributes, these are declared as part of the database schema, on par with tables.

  • Assertion

    A boolean-valued SQL expression that must remain true at all times. The database state is always expected to satisfy this condition.

  • Trigger

    A series of actions associated with specific events, such as an INSERT or UPDATE on a particular relation. These actions are performed whenever the triggering events occur.

Definition: Assertion

An assertion is a predicate expressing a condition that the database is required to satisfy. It is a boolean-valued SQL expression that must evaluate to true for the database to be in a consistent state. Any data modification operation that causes the assertion’s condition to become false is rejected. The standard SQL syntax for creating an assertion is:

CREATE ASSERTION <assertion-name> CHECK (<condition>)

Assertions offer a declarative way to enforce complex, multi-table constraints. However, there is a fundamental trade-off between the declarative nature of assertions and the procedural nature of triggers.

  • Ease of Use

    Assertions are generally easier for a programmer to formulate. One simply states a proposition that must be true. The DBMS is then responsible for enforcing it.

  • Implementation Efficiency

    Triggers are the feature more commonly implemented in commercial DBMSs. The reason is that implementing assertions efficiently is a difficult problem. The DBMS must deduce whether any given database modification could possibly violate the assertion. In contrast, a trigger explicitly tells the DBMS when to perform a check, which is a much simpler task to manage.

  • Version: 1
  • Model: gemini-2.5-pro
  • Created: 2025-10-15 23:19:12
  • Duration: 15.343639850616455
  • Cummulative cost: $1.49e-02

Creating Assertions

  • Describes the standard SQL syntax for creating an assertion: CREATE ASSERTION <assertion-name> CHECK (<condition>).
#2

read: Section 7.4.1: Creating Assertions (section)

model: pro
rules: bullet definition math presentation short


Present the following.  Use definition formal definition when it applies.

- Describes the standard SQL syntax for creating an assertion: `CREATE ASSERTION <assertion-name> CHECK (<condition>)`.

In the SQL standard, an assertion is declared using the CREATE ASSERTION statement. This allows for the enforcement of any condition that can be expressed in a WHERE clause.

Definition: Creating an Assertion

The standard SQL syntax for creating an assertion is as follows:

CREATE ASSERTION <assertion-name>
CHECK (<condition>)

  • The <assertion-name> provides a unique identifier for the constraint within the database schema.

  • The <condition> is a boolean-valued SQL expression that must be true at all times.

    Any data modification (INSERT, UPDATE, DELETE) that would cause the <condition> to become false will be rejected by the DBMS, thus preserving the integrity of the database. The condition can involve multiple tables and complex subqueries.

  • Version: 1
  • Model: gemini-2.5-pro
  • Created: 2025-10-15 23:19:21
  • Duration: 9.347315073013306
  • Cummulative cost: $1.03e-02

Using Assertions

  • Explains that an assertion’s condition must remain true; any modification causing it to become false will be rejected.
  • Highlights the difference between tuple-based CHECK constraints and assertions: assertions have no privileged access to a particular relation’s attributes and must introduce them through a select-from-where expression.
  • Discusses methods for creating a single true/false value for the condition, such as using NOT EXISTS on a query or applying an aggregate operator like SUM and comparing it to a constant.
#3

read: Section 7.4.2: Using Assertions (section)

model: pro
rules: bullet definition math presentation short


Present the following.  Use definition formal definition when it applies.

- Explains that an assertion's condition must remain true; any modification causing it to become false will be rejected.
- Highlights the difference between tuple-based CHECK constraints and assertions: assertions have no privileged access to a particular relation's attributes and must introduce them through a select-from-where expression.
- Discusses methods for creating a single true/false value for the condition, such as using `NOT EXISTS` on a query or applying an aggregate operator like `SUM` and comparing it to a constant.

  • An assertion’s condition must hold true when it is created and must remain true throughout the life of the database.

    Any database modification that results in the condition evaluating to false will be rejected. This ensures the database state remains consistent with the declared rule.

  • Assertions differ from tuple-based CHECK constraints in how they reference attributes.

    A tuple-based CHECK constraint can refer directly to the attributes of the relation in which it is declared. An assertion has no such privilege. It must introduce any referenced attributes and their corresponding relations through a select-from-where expression.

  • The condition of an assertion must produce a single boolean value.

    This is typically achieved in one of two ways. First, one can write a query that identifies any state that violates the desired condition and then apply the NOT EXISTS operator. The assertion is satisfied if this query always returns an empty set. Second, one may apply an aggregate operator, such as SUM, to a column and compare the result to a constant value.

  • Version: 1
  • Model: gemini-2.5-pro
  • Created: 2025-10-15 23:19:32
  • Duration: 10.939481258392334
  • Cummulative cost: $1.25e-02