SQL Server: Difference between PARTITION BY and GROUP BY
They're used in different places. group by
modifies the entire query, like:
select customerId, count(*) as orderCountfrom Ordersgroup by customerId
But partition by
just works on a window function, like row_number
:
select row_number() over (partition by customerId order by orderId) as OrderNumberForThisCustomerfrom Orders
A group by
normally reduces the number of rows returned by rolling them up and calculating averages or sums for each row. partition by
does not affect the number of rows returned, but it changes how a window function's result is calculated.
We can take a simple example.
Consider a table named TableA
with the following values:
id firstname lastname Mark-------------------------------------------------------------------1 arun prasanth 402 ann antony 453 sruthy abc 416 new abc 471 arun prasanth 451 arun prasanth 492 ann antony 49
GROUP BY
The SQL GROUP BY clause can be used in a SELECT statement to collect data across multiple records and group the results by one or more columns.
In more simple words GROUP BY statement is used in conjunction with the aggregate functions to group the result-set by one or more columns.
Syntax:
SELECT expression1, expression2, ... expression_n, aggregate_function (aggregate_expression)FROM tablesWHERE conditionsGROUP BY expression1, expression2, ... expression_n;
We can apply GROUP BY
in our table:
select SUM(Mark)marksum,firstname from TableAgroup by id,firstName
Results:
marksum firstname----------------94 ann 134 arun 47 new 41 sruthy
In our real table we have 7 rows and when we apply GROUP BY id
, the server group the results based on id
:
In simple words:
here
GROUP BY
normally reduces the number of rows returned by rolling them up and calculatingSum()
for each row.
PARTITION BY
Before going to PARTITION BY, let us look at the OVER
clause:
According to the MSDN definition:
OVER clause defines a window or user-specified set of rows within a query result set. A window function then computes a value for each row in the window. You can use the OVER clause with functions to compute aggregated values such as moving averages, cumulative aggregates, running totals, or a top N per group results.
PARTITION BY will not reduce the number of rows returned.
We can apply PARTITION BY in our example table:
SELECT SUM(Mark) OVER (PARTITION BY id) AS marksum, firstname FROM TableA
Result:
marksum firstname -------------------134 arun 134 arun 134 arun 94 ann 94 ann 41 sruthy 47 new
Look at the results - it will partition the rows and returns all rows, unlike GROUP BY.
partition by
doesn't actually roll up the data. It allows you to reset something on a per group basis. For example, you can get an ordinal column within a group by partitioning on the grouping field and using rownum()
over the rows within that group. This gives you something that behaves a bit like an identity column that resets at the beginning of each group.