How to round an average to 2 decimal places in PostgreSQL?
SqlRubyPostgresqlSequelSql Problem Overview
I am using PostgreSQL via the Ruby gem 'sequel'.
I'm trying to round to two decimal places.
Here's my code:
SELECT ROUND(AVG(some_column),2)
FROM table
I get the following error:
PG::Error: ERROR: function round(double precision, integer) does
not exist (Sequel::DatabaseError)
I get no error when I run the following code:
SELECT ROUND(AVG(some_column))
FROM table
Does anyone know what I am doing wrong?
Sql Solutions
Solution 1 - Sql
PostgreSQL does not define round(double precision, integer)
. For reasons @Mike Sherrill 'Cat Recall' explains in the comments, the version of round that takes a precision is only available for numeric
.
regress=> SELECT round( float8 '3.1415927', 2 );
ERROR: function round(double precision, integer) does not exist
regress=> \df *round*
List of functions
Schema | Name | Result data type | Argument data types | Type
------------+--------+------------------+---------------------+--------
pg_catalog | dround | double precision | double precision | normal
pg_catalog | round | double precision | double precision | normal
pg_catalog | round | numeric | numeric | normal
pg_catalog | round | numeric | numeric, integer | normal
(4 rows)
regress=> SELECT round( CAST(float8 '3.1415927' as numeric), 2);
round
-------
3.14
(1 row)
(In the above, note that float8
is just a shorthand alias for double precision
. You can see that PostgreSQL is expanding it in the output).
You must cast the value to be rounded to numeric
to use the two-argument form of round
. Just append ::numeric
for the shorthand cast, like round(val::numeric,2)
.
If you're formatting for display to the user, don't use round
. Use to_char
(see: data type formatting functions in the manual), which lets you specify a format and gives you a text
result that isn't affected by whatever weirdness your client language might do with numeric
values. For example:
regress=> SELECT to_char(float8 '3.1415927', 'FM999999999.00');
to_char
---------------
3.14
(1 row)
to_char
will round numbers for you as part of formatting. The FM
prefix tells to_char
that you don't want any padding with leading spaces.
Solution 2 - Sql
((this is a Wiki! please edit to enhance!))
Try also the old syntax for casting,
SELECT ROUND( AVG(some_column)::numeric, 2 ) FROM table;
works with any version of PostgreSQL.
...But, as definitive solution, you can overload the ROUND function.
Overloading as casting strategy
CREATE FUNCTION ROUND(float,int) RETURNS NUMERIC AS $f$
SELECT ROUND( CAST($1 AS numeric), $2 )
$f$ language SQL IMMUTABLE;
Now your instruction will works fine, try this complete comparison:
SELECT trunc(n,3), round(n,3) n_round, round(f,3) f_round,
pg_typeof(n) n_type, pg_typeof(f) f_type, pg_typeof(round(f,3)) f_round_type
FROM (SELECT 2.0/3.0, 2/3::float) t(n,f);
trunc | n_round | f_round | n_type | f_type | f_round_type |
---|---|---|---|---|---|
0.666 | 0.667 | 0.667 | numeric | double precision | numeric |
The ROUND(float,int) function is f_round
, it returns a (decimal) NUMERIC datatype, that is fine for some applications: problem solved!
In another applications we need a float also as result. An alternative is to use round(f,3)::float
or to create a round_tofloat()
function.
Other alternative, overloading ROUND
function again, and using all range of accuracy-precision of a floating point number, is to return a float when the accuracy is defined (see IanKenney's answer),
CREATE FUNCTION ROUND(
input float, -- the input number
accuracy float -- accuracy, the "counting unit"
) RETURNS float AS $f$
SELECT ROUND($1/accuracy)*accuracy
$f$ language SQL IMMUTABLE;
Try
SELECT round(21.04, 0.05); -- 21.05 float!
SELECT round(21.04, 5::float); -- 20
SELECT round(1/3., 0.0001); -- 0.3333
SELECT round(2.8+1/3., 0.5); -- 3.15
SELECT round(pi(), 0.0001); -- 3.1416
PS: the command \df round
, on psql
after overloadings, will show something like this table
Schema | Name | Result | Argumentwhere float is synonymous of
------------+-------+---------+------------------ myschema | round | numeric | float, int myschema | round | float | float, float pg_catalog | round | float | float
pg_catalog | round | numeric | numeric
pg_catalog | round | numeric | numeric, int
double precision
and myschema is public
when you not use a schema. The pg_catalog
functions are the default ones, see at Guide the build-in math functions.
Rounding and formating
The to_char
function apply internally the round procedure, so, when your aim is only to show a final result in the terminal, you can use the FM
modifier as a prefix to a numeric format pattern:
SELECT round(x::numeric,2), trunc(x::numeric,2), to_char(x, 'FM99.99')
FROM (SELECT 2.0/3) t(x);
round | trunc | to_char |
---|---|---|
0.67 | 0.66 | .67 |
NOTES
Cause of the problem
There are a lack of overloads in some PostgreSQL functions, why (???): I think "it is a lack" (!), but @CraigRinger, @Catcall and the PostgreSQL team agree about "pg's historic rationale".
Note about performance and reuse
The build-in functions, such as ROUND of the pg_catalog, can be overloaded with no performance loss, when compared to direct cast encoding. Two precautions must be taken when implementing user-defined cast functions for high performance:
-
The
IMMUTABLE
clause is very important for code snippets like this, because, as said in the Guide: "allows the optimizer to pre-evaluate the function when a query calls it with constant arguments" -
PLpgSQL is the preferred language, except for "pure SQL". For JIT optimizations (and sometimes for parallelism)
language SQL
can obtain better optimizations. Is something like copy/paste small piece of code instead of use a function call.
Conclusion: the above ROUND(float,int)
function, after optimizations, is so fast than @CraigRinger's answer; it will compile to (exactly) the same internal representation. So, although it is not standard for PostgreSQL, it can be standard for your projects, by a centralized and reusable "library of snippets", like pg_pubLib.
Round to the nth bit or other numeric representation
Some people argue that it doesn't make sense for PostgreSQL to round a number of float datatype, because float is a binary representation, it requires rounding the number of bits or its hexadecimal representation.
Well, let's solve the problem, adding an exotic suggestion... The aim here is to return a float type in another overloaded function,
ROUND(float, text, int) RETURNS float
The text
is to offer a choice between
'dec'
for "decimal representation",'bin'
for "binary" representation and'hex'
for hexadecimal representation.
So, in different representations we have a different interpretation about the number of digits to be rounded. Rounding a number x with an approximate shorter value, with less "fractionary digits" (tham its original d digits), will be shorter when d is couting binary digits instead decimal or hexadecimal.
It is not easy without C++, using "pure SQL", but this code snippets will illustrate and can be used as workaround:
-- Looking for a round_bin() function! this is only a workaround:
CREATE FUNCTION trunc_bin(x bigint, t int) RETURNS bigint AS $f$
SELECT ((x::bit(64) >> t) << t)::bigint;
$f$ language SQL IMMUTABLE;
CREATE FUNCTION ROUND(
x float,
xtype text, -- 'bin', 'dec' or 'hex'
xdigits int DEFAULT 0
)
RETURNS FLOAT AS $f$
SELECT CASE
WHEN xtype NOT IN ('dec','bin','hex') THEN 'NaN'::float
WHEN xdigits=0 THEN ROUND(x)
WHEN xtype='dec' THEN ROUND(x::numeric,xdigits)
ELSE (s1 ||'.'|| s2)::float
END
FROM (
SELECT s1,
lpad(
trunc_bin( s2::bigint, CASE WHEN xd<bin_bits THEN bin_bits - xd ELSE 0 END )::text,
l2,
'0'
) AS s2
FROM (
SELECT *,
(floor( log(2,s2::numeric) ) +1)::int AS bin_bits, -- most significant bit position
CASE WHEN xtype='hex' THEN xdigits*4 ELSE xdigits END AS xd
FROM (
SELECT s[1] AS s1, s[2] AS s2, length(s[2]) AS l2
FROM (SELECT regexp_split_to_array(x::text,'\.')) t1a(s)
) t1b
) t1c
) t2
$f$ language SQL IMMUTABLE;
Try
SELECT round(1/3.,'dec',4); -- 0.3333 float!
SELECT round(2.8+1/3.,'dec',1); -- 3.1 float!
SELECT round(2.8+1/3.,'dec'); -- ERROR, need to cast string
SELECT round(2.8+1/3.,'dec'::text); -- 3 float
SELECT round(2.8+1/3.,'dec',0); -- 3 float
SELECT round(2.8+1/3.,'hex',0); -- 3 float (no change)
SELECT round(2.8+1/3.,'hex',1); -- 3.1266
SELECT round(2.8+1/3.,'hex',3); -- 3.13331578486784
SELECT round(2.8+1/3.,'bin',1); -- 3.1125899906842625
SELECT round(2.8+1/3.,'bin',6); -- 3.1301821767286784
SELECT round(2.8+1/3.,'bin',12); -- 3.13331578486784
And \df round
have also:
Schema | Name | Result | Argument
------------+-------+---------+---------------
myschema | round | float | x float, xtype text, xdigits int DEFAULT 0
Solution 3 - Sql
Try with this:
SELECT to_char (2/3::float, 'FM999999990.00');
-- RESULT: 0.67
Or simply:
SELECT round (2/3::DECIMAL, 2)::TEXT
-- RESULT: 0.67
Solution 4 - Sql
you can use the function below
SELECT TRUNC(14.568,2);
the result will show :
14.56
you can also cast your variable to the desire type :
SELECT TRUNC(YOUR_VAR::numeric,2)
Solution 5 - Sql
Try casting your column to a numeric like:
SELECT ROUND(cast(some_column as numeric),2) FROM table
Solution 6 - Sql
SELECT ROUND(SUM(amount)::numeric, 2) AS total_amount
FROM transactions
Gives: 200234.08
Solution 7 - Sql
According to Bryan's response you can do this to limit decimals in a query. I convert from km/h to m/s and display it in dygraphs but when I did it in dygraphs it looked weird. Looks fine when doing the calculation in the query instead. This is on postgresql 9.5.1.
select date,(wind_speed/3.6)::numeric(7,1) from readings;
Solution 8 - Sql
> Error:function round(double precision, integer) does not exist
Solution: You need to addtype cast then it will work
Ex: round(extract(second from job_end_time_t)::integer,0)