Define the term cardinality in SQL

What is cardinality in SQL, define cardinality, what is the cardinality of a relational table, what is the cardinality of an attribute

Cardinality (in SQL)

In SQL (Structured Query Language), the term cardinality means the uniqueness of the data values that are stored in a table. It can be defined as follows;

• Cardinality of a relation (table) – the number of records (tuples) in the given table.

• Cardinality of an attribute (column) – the number of unique data values in the specified attribute.

See the example EMPLOYEES table with few records below;

EMPLOYEES Table
EMPID	HIREDATE	SALARY	DEPT	JOBCODE	SEX
119012	01JUL1973	42340.58	CSR010	602	F
120591	05DEC1985	31000.55	SHP002	602	F
127845	16JAN1972	75320.34	ACC024	204	M
129540	01AUG1987	56123.34	SHP002	204	F
135673	15JUL1989	46322.58	ACC013	602	F
212916	15FEB1958	52345.58	CSR010	602	F
216382	15JUN1990	34004.65	SHP013	602	F
234967	19DEC1993	17000.00	CSR004	602	M
237642	01NOV1981	43200.34	SHP013	602	M
239185	07MAY1986	57920.66	ACC024	602	M

• Cardinality of the table

• EMPLOYEES table – 10 (10 records are there in EMPLOYEES table)

• Cardinality of the attributes

• EMPID attribute – 10 (only 10 employees listed in this table).

• DEPT attribute – 6 (only 6 departments – in other words only 6 permitted values for DEPT attribute)

• JOBCODE attribute – 2 (only 2 types of jobs – in other words only 2 permitted values for JOBCODE attribute)

• SEX attribute – 2 (only 2 genders male and female – in other words only 2 permitted values for SEX attribute)

And so on.

Note: Cardinality of the PRIMARY KEY attribute is equal to the number of records in the table. For example, EMPID is the primary key and hence the unique values are equal to the number of records, ie. 10.

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