Notnullrajotteet

Notnullrajotteet are a type of database constraint that ensures a column cannot contain NULL values. They enforce that data must be present for the constrained column in every row.

Scope and purpose: Notnullrajotteet apply to individual columns and are commonly used to guarantee that essential fields, such as identifiers or names, always have a value. They can be defined when a table is created or added later with an ALTER TABLE statement.

Syntax and examples: In SQL, a NOT NULL constraint is typically declared as part of a column definition, for example: id INT PRIMARY KEY, name VARCHAR(100) NOT NULL. Some databases also support separate constraint syntax, but the standard approach is to declare NOT NULL on the column. The exact ALTER syntax varies by database system: PostgreSQL uses ALTER TABLE table ALTER COLUMN column SET NOT NULL; MySQL uses ALTER TABLE table MODIFY COLUMN column datatype NOT NULL; SQL Server uses ALTER TABLE table ALTER COLUMN column datatype NOT NULL.

Handling existing data: If a column already contains NULL values, adding a NOT NULL constraint will fail until those NULLs are removed or replaced. In such cases, you must update or backfill the NULL values or temporarily remove or defer the constraint during migration.

Relation to other constraints: NOT NULL is distinct from, but commonly used with, PRIMARY KEY and UNIQUE constraints. It can be complemented by CHECK constraints to express more complex rules about allowed values.

Best practices: Apply notnullrajotteet to columns that must always have a value, use sensible defaults where appropriate, and plan migrations carefully to preserve data integrity and minimize downtime. The concept is a standard feature across relational databases and generally incurs minimal performance impact beyond the constraint checks themselves.