Page 89 - SQL

You can count the number of rows:

SELECT count(*) TotalRows
FROM employees;

TotalRows

4

Or count the employees per department:

SELECT DepartmentId, count(*) NumEmployees
FROM employees
GROUP BY DepartmentId;

DepartmentId NumEmployees

1 3

2 1

You can count over a column/expression with the effect that will not count the NULL values:

SELECT count(ManagerId) mgr
FROM EMPLOYEES;

mgr

3

(There is one null value managerID column)

You can also use DISTINCT inside of another function such as COUNT to only find the DISTINCT
members of the set to perform the operation on.

For example:

SELECT COUNT(ContinentCode) AllCount
, COUNT(DISTINCT ContinentCode) SingleCount
FROM Countries;

Will return different values. The SingleCount will only Count individual Continents once, while the
AllCount will include duplicates.

ContinentCode

OC

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