Grouping and Having

Introduzione a SQL Server

John MacKintosh

Instructor

A simple SELECT

SELECT 
  SUM(demand_loss_mw) AS lost_demand 
FROM grid;

+-------------+
| lost_demand | 
+-------------+
| 177888      |
+-------------+

Grouping error

Can we break this down by adding an additional column?

SELECT 
  SUM(demand_loss_mw) AS lost_demand, 
  description 
FROM grid;

Msg 8120, Level 16, State 1, Line 1
Column 'grid.description' is invalid in the select list because it is not contained in 
either an aggregate function or the GROUP BY clause.

SELECT 
  SUM(demand_loss_mw) AS lost_demand,
  description 
FROM grid 
GROUP BY description;

+----------- -+-------------------------------------------------------+
| lost_demand | description                                           | 
+-------------+-------------------------------------------------------+
| NULL        | Actual Physical Attack                                |
| NULL        | Cold Weather Event                                    |
| NULL        | Cyber Event with Potential to Cause Impact            |
| 40          | Distribution Interruption                             |
| 2           | Distribution System Interruption                      |
| NULL        | Earthquake                                            |
| NULL        | Electrical Fault at Generator                         |
| 338         | Electrical System Islanding                           |
| 24514       | Electrical System Separation Islanding                |
| 15          | Electrical System Separation Islanding Severe Weather |
+-------------+-------------------------------------------------------+

SELECT 
  SUM(demand_loss_mw) AS lost_demand, 
  description 
FROM grid 
WHERE 
  description LIKE '%storm' 
  AND demand_loss_mw IS NOT NULL 
GROUP BY description;

+----------- -+--------------------------------------------+
| lost_demand | description                                | 
|-------------+--------------------------------------------|
| 996         | Ice Storm                                  |
| 420         | Load Shed Severe Weather   Lightning Storm |
| 332         | Major Storm                                |
| 3           | Severe Weather  Thunderstorm               |
| 413         | Severe Weather  Wind Storm                 |
| 4171        | Severe Weather  Winter Storm               |
| 1352        | Winter Storm                               |
+-------------+--------------------------------------------+

HAVING

Can use aggregate functions in SELECT
Filter data using WHERE
Split data into groups using GROUP BY
What if we want to sum values based on groups?
... and then filter on those sums?

SELECT 
  SUM(demand_loss_mw) AS lost_demand, 
  description 
FROM grid 
WHERE 
  description LIKE '%storm' 
  AND demand_loss_mw IS NOT NULL 
GROUP BY description;

+----------- -+--------------------------------------------+
| lost_demand | description                                | 
+-------------+--------------------------------------------+
| 996         | Ice Storm                                  |
| 420         | Load Shed Severe Weather   Lightning Storm |
| 332         | Major Storm                                |
| 3           | Severe Weather  Thunderstorm               |
| 413         | Severe Weather  Wind Storm                 |
| 4171        | Severe Weather  Winter Storm               |
| 1352        | Winter Storm                               |
+-------------+--------------------------------------------+

SELECT 
  SUM(demand_loss_mw) AS lost_demand, 
  description 
FROM grid 
WHERE 
  description LIKE '%storm' 
  AND demand_loss_mw IS NOT NULL 
GROUP BY description
HAVING SUM(demand_loss_mw) > 1000;

+----------- -+--------------------------------------------+
| lost_demand | description                                | 
|-------------+--------------------------------------------|
| 4171        | Severe Weather  Winter Storm               |
| 1352        | Winter Storm                               |
+-------------+--------------------------------------------+

Summary

GROUP BY splits the data up into combinations of one or more values
WHERE filters on row values
HAVING appears after the GROUP BY clause and filters on groups or aggregates

Let's put our skills to the test!

Introduzione a SQL Server