Cache and Raid performance. One of the key limitations of spinning disk hardware is the use of a physical reading armature and the time this takes to move across the surface of a diskâs platter to the required data blocks.
Constraints are part of a database schema definition. A constraint is usually associated with a table and is created with a CREATE CONSTRAINT or CREATE ASSERTION SQL statement. They define certain properties that data in a database must comply with.
Columns that are referenced in a unique constraint must be defined as NOT NULL. The unique constraint can be defined in the CREATE TABLE statement using the UNIQUE clause (Figure 1 and Figure 2), or in an ALTER TABLE statement, as shown in Listing 2. Listing 2 shows how to create a unique constraint.