Keys – A key refers to an attribute/a set of attributes that help us identify a row (or tuple) uniquely in a table (or relation). A key is also used when we want to establish relationships between the different columns and tables of a relational database.it helps to identify a table’s record.
Super key – these are key/keys which using we can uniquely identify any table’s record uniquely.
Candidate key – candidate key is an alternative key of primary key. Using this key we can identify any table’s record uniquely. In table/relation candidate keys may be one or many attributes.
Composite key – using this key we can identify any table’s record uniquely. In table/relation these are attributes which helps to do this. We need at-least two attributes for composite keys.
Primary key – using this key we can identify any table’s record uniquely.
Foreign key – if there are two relations/tables which have the same primary key relation, then the master table’s primary key foreign key will be another child table primary key.
Primary key is always one/single and it can’t be a NULL state.
Super key can be one or many in the table as attributes.
Composite key will be at-least two attributes in a table/relation.
Candidate key can be one or many.
Constraints – these are some rules which apply on attributes of a relational table, to restrict user’s input data.
There is much type of constraints there –
NOT NULL constraint – if we restrict any table’s field can’t be empty, then we can use this constraint.
Unique constraint – if we restrict any table field need to maintain unique value each time, then we could use this constraint.
Default constraint – if we want any field get default value each time.
Check constraint – it checks some conditions depending upon user inputs.
Read more: http://dbms.microcodes.in/index.php/basics/keys-concepts/
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