Q.Using the Employee table, write SQL queries to find

This applies the aggregate functions COUNT, SUM, AVG, MAX and MIN to the whole table.

Each part uses an aggregate function over the whole Employee table.

(i) Number of employees — COUNT(*) counts all rows:

SELECT COUNT(*) AS TotalEmployees FROM Employee;

This gives 6.

(ii) Total and average salary — SUM and AVG of the Salary column. Both ignore the NULL salary, so the average is over the five known salaries:

SELECT SUM(Salary) AS TotalSalary, AVG(Salary) AS AverageSalary
FROM Employee;

The five known salaries are 25000 + 30000 + 28000 + 32000 + 40000 = 155000, so SUM is 155000 and AVG is 155000 / 5 = 31000.

(iii) Highest and lowest salary — MAX and MIN:

SELECT MAX(Salary) AS Highest, MIN(Salary) AS Lowest
FROM Employee;
``` …

All three summaries can be combined into a single query: `SELECT COUNT(*), SUM(Salary), AVG(Salary) …

Remember AVG divides by the number of non-NULL salaries (5), not by the total number of rows (6). Writing COUNT(Salary) instead of COUNT(*) in part …