STDDEV_SAMP and STDDEV_POP window functions
The STDDEV_SAMP and STDDEV_POP window functions return the sample and population standard deviation of a set of numeric values (integer, decimal, or floating-point). See also STDDEV_SAMP and STDDEV_POP functions.
STDDEV_SAMP and STDDEV are synonyms for the same function.
Syntax
STDDEV_SAMP | STDDEV | STDDEV_POP ( [ ALL ] expression ) OVER ( [ PARTITION BY expr_list ] [ ORDER BY order_list frame_clause ] )
Arguments
- expression
-
The target column or expression that the function operates on.
- ALL
-
With the argument ALL, the function retains all duplicate values from the expression. ALL is the default. DISTINCT is not supported.
- OVER
-
Specifies the window clauses for the aggregation functions. The OVER clause distinguishes window aggregation functions from normal set aggregation functions.
- PARTITION BY expr_list
-
Defines the window for the function in terms of one or more expressions.
- ORDER BY order_list
-
Sorts the rows within each partition. If no PARTITION BY is specified, ORDER BY uses the entire table.
- frame_clause
-
If an ORDER BY clause is used for an aggregate function, an explicit frame clause is required. The frame clause refines the set of rows in a function's window, including or excluding sets of rows within the ordered result. The frame clause consists of the ROWS keyword and associated specifiers. See Window function syntax summary.
Data types
The argument types supported by the STDDEV functions are SMALLINT, INTEGER, BIGINT, NUMERIC, DECIMAL, REAL, and DOUBLE PRECISION.
Regardless of the data type of the expression, the return type of a STDDEV function is a double precision number.
Examples
The following example shows how to use STDDEV_POP and VAR_POP functions as window functions. The query computes the population variance and population standard deviation for PRICEPAID values in the SALES table.
select salesid, dateid, pricepaid,
round(stddev_pop(pricepaid) over
(order by dateid, salesid rows unbounded preceding)) as stddevpop,
round(var_pop(pricepaid) over
(order by dateid, salesid rows unbounded preceding)) as varpop
from sales
order by 2,1;
salesid | dateid | pricepaid | stddevpop | varpop
--------+--------+-----------+-----------+---------
33095 | 1827 | 234.00 | 0 | 0
65082 | 1827 | 472.00 | 119 | 14161
88268 | 1827 | 836.00 | 248 | 61283
97197 | 1827 | 708.00 | 230 | 53019
110328 | 1827 | 347.00 | 223 | 49845
110917 | 1827 | 337.00 | 215 | 46159
150314 | 1827 | 688.00 | 211 | 44414
157751 | 1827 | 1730.00 | 447 | 199679
165890 | 1827 | 4192.00 | 1185 | 1403323
...The sample standard deviation and variance functions can be used in the same way.
