Database Constraint is user-defined structures that express the behaviors of column. It is used to protect wrong entry. There are five types of database constraints. Such
Primary Key Constraint
A column value will be unique among all the rows in a table. It does not allow any null value. Therefore, a primary key constraint has the behaviors of both Not Null and Unique Key Constraints.
Foreign Key Constraint
We can’t insert a foreign key value which is not present in the primary key column of that foreign key.
Unique Key Constraint
A column value will be unique among all the rows in a table. It allows only a single Null value in that column. Depending on the design, a table may have more than one unique key constraint.
Not null Constraint
It makes a column mandatory. This means you can’t insert a row in the table without providing a valid data.
Check Constraint
Check Constraints are table-level constraint. It doesn’t make the column mandatory.
- Primary Key Constraint
- Foreign Key Constraint
- Unique Key Constraint
- Not null Constraint
- Check Constraint
Primary Key Constraint
A column value will be unique among all the rows in a table. It does not allow any null value. Therefore, a primary key constraint has the behaviors of both Not Null and Unique Key Constraints.
Foreign Key Constraint
We can’t insert a foreign key value which is not present in the primary key column of that foreign key.
Unique Key Constraint
A column value will be unique among all the rows in a table. It allows only a single Null value in that column. Depending on the design, a table may have more than one unique key constraint.
Not null Constraint
It makes a column mandatory. This means you can’t insert a row in the table without providing a valid data.
Check Constraint
Check Constraints are table-level constraint. It doesn’t make the column mandatory.
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