what are different types of constraint?
Answers
Answer:
There are five types of constraints:
A NOT NULL constraint is a rule that prevents null values from being entered into one or more columns within a table.
A unique constraint (also referred to as a unique key constraint) is a rule that forbids duplicate values in one or more columns within a table. Unique and primary keys are the supported unique constraints. For example, a unique constraint can be defined on the supplier identifier in the supplier table to ensure that the same supplier identifier is not given to two suppliers.
A primary key constraint is a column or combination of columns that has the same properties as a unique constraint. You can use a primary key and foreign key constraints to define relationships between tables.
A foreign key constraint (also referred to as a referential constraint or a referential integrity constraint) is a logical rule about values in one or more columns in one or more tables. For example, a set of tables shares information about a corporation's suppliers. Occasionally, a supplier's name changes. You can define a referential constraint stating that the ID of the supplier in a table must match a supplier ID in the supplier information. This constraint prevents insert, update, or delete operations that would otherwise result in missing supplier information.
A (table) check constraint (also called a check constraint) sets restrictions on data added to a specific table. For example, a table check constraint can ensure that the salary level for an employee is at least $20 000 whenever salary data is added or updated in a table containing personnel information.
Explanation:
An informational constraint is an attribute of a certain type of constraint, but one that is not enforced by the database manager.
NOT NULL constraints
NOT NULL constraints prevent null values from being entered into a column.
Unique constraints
Unique constraints ensure that the values in a set of columns are unique and not null for all rows in the table. The columns specified in a unique constraint must be defined as NOT NULL. The database manager uses a unique index to enforce the uniqueness of the key during changes to the columns of the unique constraint.
Primary key constraints
You can use primary key and foreign key constraints to define relationships between tables.
(Table) Check constraints
A check constraint (also referred to as a table check constraint) is a database rule that specifies the values allowed in one or more columns of every row of a table. Specifying check constraints is done through a restricted form of a search condition.
Foreign key (referential) constraints
Foreign key constraints (also known as referential constraints or referential integrity constraints) enable definition of required relationships between and within tables.
Informational constraints
An informational constraint is a constraint attribute that can be used by the SQL compiler to improve the access to data. Informational constraints are not enforced by the database manager, and are not used for additional verification of data; rather, they are used to improve query performance.
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