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## 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;

 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.