Group by (SQL)
A GROUP BY statement in SQL specifies that a SQL SELECT statement partitions result rows into groups, based on their values in one or several columns. Typically, grouping is used to apply some sort of aggregate function for each group.{{Cite web|title=SQL GROUP BY Statement|url=https://www.w3schools.com/sql/sql_groupby.asp|access-date=2020-09-18|website=www.w3schools.com}}{{Cite web|last=shkale-msft|title=GROUP BY (Transact-SQL) - SQL Server|url=https://docs.microsoft.com/en-us/sql/t-sql/queries/select-group-by-transact-sql|access-date=2020-09-18|website=docs.microsoft.com|language=en-us}}
The result of a query using a GROUP BY statement contains one row for each group. This implies constraints on the columns that can appear in the associated SELECT clause. As a general rule, the SELECT clause may only contain columns with a unique value per group. This includes columns that appear in the GROUP BY clause as well as aggregates resulting in one value per group.{{Cite web|title=SQL Grouping and Aggregation|url=http://www.databaselecture.com/sql.html|access-date=2020-12-09|website=databaselecture.com|language=en-us}}
Examples
Returns a list of Department IDs along with the sum of their sales for the date of January 1, 2000.
SELECT DeptID, SUM(SaleAmount) FROM Sales
WHERE SaleDate = '01-Jan-2000'
GROUP BY DeptID
In the following example one can ask "How many units were sold in each region for every ship date?":
| class="wikitable"
!Sum of units !Ship date ▼ |
| Region ▼
!2005-01-31 !2005-02-28 !2005-03-31 !2005-04-30 !2005-05-31 !2005-06-30 |
|---|
| East
|66 |80 |102 |116 |127 |125 |
| North
|96 |117 |138 |151 |154 |156 |
| South
|123 |141 |157 |178 |191 |202 |
| West
|78 |97 |117 |136 |150 |157 |
| (blank)
| | | | | | |
| Grand total
|363 |435 |514 |581 |622 |640 |
The following code returns the data of the above pivot table which answers the question "How many units were sold in each region for every ship date?":
SELECT Region, Ship_Date, SUM(Units) AS Sum_of_Units
FROM FlatData
GROUP BY Region, Ship_Date
WITH ROLLUP
Since SQL:1999, GROUP BY can be extended WITH ROLLUP to add a result line with a super-aggregator result. In the above example, it corresponds to the Grand total line.
Common groupings
Common grouping (aggregation) functions include:
- Count(expression) - Quantity of matching records (per group)
- Sum(expression) - Summation of given value (per group)
- Min(expression) - Minimum of given value (per group)
- Max(expression) - Maximum of given value (per group)
- Avg(expression) - Average of given value (per group)
See also
References
External links
- [http://www.sqlsnippets.com/en/topic-13100.html SQL Snippets: SQL Features Tutorials - Grouping Rows with GROUP BY]
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Category:Articles with example SQL code
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