Question

How does SQL allow implementation of general integrity constraints?

Short answer

SQL allows implementation of general integrity constraints through the use of domain, entity, and referential integrity along with Check Constraints. Domain integrity is integrated by defining the data type for the columns, and entity integrity by defining unique primary key values for each row. Referential integrity is done using foreign keys that relate to primary keys in other tables, and check constraints are used to set specific conditions for the data that needs to be fulfilled.

Step-by-step solution

Understand SQL Integrity Constraints

Integrity constraints are rules that help to ensure the quality of the information stored in the database. The three types of integrity constraints predominantly used in SQL are Domain, Entity and Referential integrity constraints.

Implementation of Domain Integrity

Domain Integrity involves checks on the type of data entered. This is usually done by defining the data type and possible values for the columns while creating the tables. Numeric columns can have checks on the range of values allowed.

Implementation of Entity Integrity

Entity Integrity involves ensuring the uniqueness of rows in a table. This can be done by defining one or more columns as Primary key, which forces every row to have a unique value for the designated columns. This can be implemented in SQL using CREATE TABLE command followed by column name and PRIMARY KEY keyword.

Implementation of Referential Integrity

Referential Integrity involves ensuring the relationship between columns in tables. Usually, it is implemented using foreign key constraints on the columns that relate to primary key columns in other tables. Foreign key constraint in SQL is usually added using FOREIGN KEY keyword followed by column name, REFERENCES keyword and respective parent table name.

Assignment of Check Constraints

Check constraints can be used to assign more specific conditions that data in the tables must fulfil. Typically, this is done using the CHECK keyword while defining the table. The CHECK constraint makes sure that all values in a column or set of columns fulfill a specific condition set by the database designer.

Key concepts

Domain Integrity

Domain integrity ensures that the data entered into a database is both compliant with predefined rules and valid in terms of the data type. This means every column has defined types and constraints. For instance, defining a column to only accept integers. Domain integrity checks suppress invalid data entries. When a table is created, we use SQL data types, such as INT, VARCHAR, or DATE, to achieve this.

• Ensure that only valid values enter a column.
• Avoids data inconsistency by using correct data types.

Besides data types, domain integrity can also enforce constraints like NOT NULL, ensuring no null values enter a particular column. Numeric ranges and valid states for a column, such as a gender column restricted to 'Male' or 'Female', are typical domain integrity examples.

Entity Integrity

Entity integrity is all about the uniqueness of records within a table. This is enforced through the use of primary keys, which ensure each row is distinct. A primary key is a column or set of columns that uniquely identifies each record in a table. In SQL, we implement this as part of the table creation process.

• A primary key cannot have NULL values.
• A table can only have one primary key, which may consist of one or more columns.

For instance, an 'Employee' table might use 'EmployeeID' as the primary key, ensuring that no two employees have the same identifier. This concept assures every entry in the database is uniquely identifiable and secure from duplication.

Referential Integrity

Referential integrity deals with the relationships between tables. It's maintained by using foreign keys, which link to primary keys in another table. This ensures that relationships between tables remain consistent. When creating or modifying tables, foreign keys are added to describe these connections.

• Foreign keys reference primary keys from other tables.
• Enforces parent-child relationships between tables.

If we have 'Orders' and 'Customers' tables, the 'Orders' table might contain a 'CustomerID' foreign key that references 'CustomerID' in the 'Customers' table. Thus, no order can exist without a valid customer, maintaining the relational aspect of the databases.

Check Constraints

Check constraints allow you to enforce specific, custom conditions on the data in a column. They provide an additional layer of data validity by ensuring data entry follows particular rules or formulas, which are defined during table creation.

• Implemented using the CHECK keyword.
• Ensures that data meets certain conditions before being inserted into the database.

For example, in a student database, you might want to ensure that a 'grade' column only accepts values between 0 and 100. By implementing a CHECK constraint, any data attempting to violate this rule would be rejected, thus ensuring greater data accuracy and reliability.