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Wednesday, June 20, 2012

SQL Group by

Group by statement tells SQL server,  pull out the distinct values for the columns specified after group by. The Select says, once you pull out the distinct values, give me this aggregate.


Name
Department
Salary
Gender
Alan
electrical
1000
M
Sue
electrical
1500
F
Sam
electrical
1400
M
Jamie
computer
2500
F
John
computer
3000
M
Sandra
HR
1100
F
Sharon
HR
1200
F



If I want to find the number of people from each department, what I will tell SQL server is to group by department.

SELECT department,
       Count(*)
FROM   employees
GROUP  BY department

The above statement can be read as, fetch distinct values of department, then tell me the number of rows in which each department appears.

The results for the above statement are
department
(No column name)
computer
2
electrical
3
HR
2



Suppose I want the maximum salary for each department, my SQL statement would be

select department,MAX(salary)
from Employees
group by
 department



department
(No column name)
computer
3000
electrical
1500
HR
1200


These are the commonly used aggregate functions in SQL

COUNT()
Returns the number of rows
AVG() 
Returns the average value
MAX() 
Returns the highest  value
MIN()
Returns the lowest value
SUM()
Returns the sum
FIRST()
Returns the first value
LAST()
Returns the last value




Check this out for a complete reference


Having Clause

When you use Group by, you cannot use where on aggregate functions. For example 

SELECT department,
       Count(*)
FROM   employees
GROUP  BY department
where Count(*) >2

will fail. For aggregate functions you can use having instead

SELECT department,
       Count(*)
FROM   employees
GROUP  BY department
having Count(*) >2







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