What do you mean by cardinality and its types?

In the context of data models, cardinality means the relationship between two tables.

The connection can be of 4 types

• One to One:-One row of the first table partners with one row of the second table
• One to Many:-One row of the first table partners with more than one rows of the second table
• Many to One:-More than one rows of the first table partners with one row of the second table
• Many to Many:-More than one rows of the first table partners with more than one rows of the second table

In the context of Query Optimization, Cardinality refers to the uniqueness of the column in the table. A column having unique value will have high cardinality and vice-versa.