Every relation in a relational database model should abide by or follow a few constraints to be a valid relation, these constraints are called as Relational Integrity Constraints.
The three main Integrity Constraints are:
- Key Constraints
- Domain Constraints
- Referential integrity Constraints
We store data in tables, to later access it whenever required. In every table one or more than one attributes together are used to fetch data from tables. The Key Constraint specifies that there should be such an attribute(column) in a relation(table), which can be used to fetch data for any tuple(row).
The Key attribute should never be NULL or same for two different row of data.
For example, in the Employee table we can use the attribute ID to fetch data for each of the employee. No value of ID is null and it is unique for every row, hence it can be our Key attribute.
Domain constraints refers to the rules defined for the values that can be stored for a certain attribute.
Like we explained above, we cannot store Address of employee in the column for Name.
Similarly, a mobile number cannot exceed 10 digits.
Referential Integrity Constraint
We will study about this in detail later. For now remember this example, if I say Supriya , then a girl with name Supriya should also exist for that relationship to be present.
If a table reference to some data from another table, then that table and that data should be present for referential integrity constraint to hold true.