Within PostgresSQL how can I define a parameter as a function, i.e.
SELECT MAX(value) FROM table;
SELECT MIN(value) FROM table;
SELECT AVG(value) FROM table;
where the function is a parameter, something like the following:
SELECT $1(value) FROM table;
You cannot do that. Use multiple separate queries, and choose the one to execute. If you have to do this within SQL, you can also do the choosing depending on a parameter:
SELECT (CASE $1::text
WHEN 'max' THEN (SELECT MAX(value) FROM table)
WHEN 'min' THEN (SELECT MIN(value) FROM table)
WHEN 'avg' THEN (SELECT AVG(value) FROM table)
END);
Related
I've got a table with a lot of columns in it and I want to run a query to find the most common value in each column.
Ordinarily for a single column, I'd run something like:
SELECT country
FROM users
GROUP BY country
ORDER BY count(*) DESC
LIMIT 1
Does PostgreSQL have a built in function for doing this or can anyone suggest a query I could run to achieve this?
Using the same query, for more than one column you should do:
SELECT *
FROM
(
SELECT country
FROM users
GROUP BY 1
ORDER BY count(*) DESC
LIMIT 1
) country
,(
SELECT city
FROM users
GROUP BY 1
ORDER BY count(*) DESC
LIMIT 1
) city
This works for any type and will return all the values in the same row, with the columns having its original name.
For more columns just had more subquerys as:
,(
SELECT someOtherColumn
FROM users
GROUP BY 1
ORDER BY count(*) DESC
LIMIT 1
) someOtherColumn
Edit:
You could reach it with window functions also. However it will not be better in performance nor in readability.
Starting from PG 9.4 there is aggregate function for this:
mode() WITHIN GROUP (ORDER BY sort_expression)
returns the most frequent input value (arbitrarily choosing the first one if there are multiple equally-frequent results)
And for earlier versions, you could create one...
CREATE OR REPLACE FUNCTION mode_array(anyarray)
RETURNS anyelement AS
$BODY$
SELECT a FROM unnest($1) a GROUP BY 1 ORDER BY COUNT(1) DESC, 1 LIMIT 1;
$BODY$
LANGUAGE SQL IMMUTABLE;
CREATE AGGREGATE mode(anyelement)(
SFUNC = array_append, --Function to call for each row. Just builds the array
STYPE = anyarray,
FINALFUNC = mode_array, --Function to call after everything has been added to array
INITCOND = '{}'--Initialize an empty array when starting
) ;
Usage: SELECT mode(column) FROM table;
If I were doing this, I'd write a query like this one:
SELECT 'country', country
FROM users
GROUP BY country
ORDER BY count(*) DESC
LIMIT 1
UNION ALL
SELECT 'city', city
FROM USERS
GROUP BY city
ORDER BY count(*) DESC
LIMIT 1
-- etc.
It should be noted this only works if all the columns are of compatible types. If they are not, you'll probably need a different solution.
This window function version will read the users table and the computed table once each. The correlated subquery version will read the users table once for each of the columns. If the columns are many as in the OPs case then my guess is that this is faster. SQL Fiddle
select distinct on (country_count, age_count) *
from (
select
country,
count(*) over(partition by country) as country_count,
age,
count(*) over(partition by age) as age_count
from users
) s
order by country_count desc, age_count desc
limit 1
I have a field called "Users", and I want to run SUM() on that field that returns the sum of all DISTINCT records. I thought that this would work:
SELECT SUM(DISTINCT table_name.users)
FROM table_name
But it's not selecting DISTINCT records, it's just running as if I had run SUM(table_name.users).
What would I have to do to add only the distinct records from this field?
Use count()
SELECT count(DISTINCT table_name.users)
FROM table_name
SQLFiddle demo
This code seems to indicate sum(distinct ) and sum() return different values.
with t as (
select 1 as a
union all
select '1'
union all
select '2'
union all
select '4'
)
select sum(distinct a) as DistinctSum, sum(a) as allSum, count(distinct a) as distinctCount, count(a) as allCount from t
Do you actually have non-distinct values?
select count(1), users
from table_name
group by users
having count(1) > 1
If not, the sums will be identical.
You can see for yourself that distinct works with the following example. Here I create a subquery with duplicate values, then I do a sum distinct on those values.
select DistinctSum=sum(distinct x), RegularSum=Sum(x)
from
(
select x=1
union All
select 1
union All
select 2
union All
select 2
) x
You can see that the distinct sum column returns 3 and the regular sum returns 6 in this example.
You can use a sub-query:
select sum(users)
from (select distinct users from table_name);
SUM(DISTINCTROW table_name.something)
It worked for me (innodb).
Description - "DISTINCTROW omits data based on entire duplicate records, not just duplicate fields." http://office.microsoft.com/en-001/access-help/all-distinct-distinctrow-top-predicates-HA001231351.aspx
;WITH cte
as
(
SELECT table_name.users , rn = ROW_NUMBER() OVER (PARTITION BY users ORDER BY users)
FROM table_name
)
SELECT SUM(users)
FROM cte
WHERE rn = 1
SQL Fiddle
Try here yourself
TEST
DECLARE #table_name Table (Users INT );
INSERT INTO #table_name Values (1),(1),(1),(3),(3),(5),(5);
;WITH cte
as
(
SELECT users , rn = ROW_NUMBER() OVER (PARTITION BY users ORDER BY users)
FROM #table_name
)
SELECT SUM(users) DisSum
FROM cte
WHERE rn = 1
Result
DisSum
9
If circumstances make it difficult to weave a "distinct" into the sum clause, it will usually be possible to add an extra "where" clause to the entire query - something like:
select sum(t.ColToSum)
from SomeTable t
where (select count(*) from SomeTable t1 where t1.ColToSum = t.ColToSum and t1.ID < t.ID) = 0
May be a duplicate to
Trying to sum distinct values SQL
As per Declan_K's answer:
Get the distinct list first...
SELECT SUM(SQ.COST)
FROM
(SELECT DISTINCT [Tracking #] as TRACK,[Ship Cost] as COST FROM YourTable) SQ
I need to run the following query to extract the values of my raster records in a specific point.
select st_value((select rast from mytable),
(select st_GeomFromText('POINT(30.424 -1.978)', 4326)))
But I encounter with the following error:
ERROR: more than one row returned by a subquery used as an expression
SQL state: 21000
It needs just one record for this function but I need to extract values of all of records.
If a subquery returns multiple rows, you must either use it in a common table expression (CTE / WITH query) and FROM alias, or use FROM (SELECT ...) alias. In this case, though, it looks like it's simpler than that:
select st_value(rast, st_GeomFromText('POINT(30.424 -1.978)', 4326))
FROM mytable;
Both subqueries appear to be unnecessary.
If you truly needed the subquery you'd write something syntactically like:
WITH sq(rast) AS ( SELECT rast FROM mytable )
SELECT st_value(rast, st_GeomFromText('POINT(30.424 -1.978)', 4326))
FROM sq;
or
SELECT st_value(rast, st_GeomFromText('POINT(30.424 -1.978)', 4326))
FROM (SELECT rast FROM mytable) sq(rast);
Try:
Select st_value(rast),
st_GeomFromText('POINT(30.424 -1.978)', 4326)
from mytable
If you have a function with multiple columns, you can do something like this
SELECT (info).column1, (info).column2, (info).column3
FROM (select st_value(rast, st_GeomFromText('POINT(30.424 -1.978)', 4326)) AS info
FROM mytable
) AS foo
How I can make/declare/define a derived column in select query and then use it in where clause?
To define a column in an SQL query, you can use pretty much any SQL operation that returns a single value (including select statements). Here are some examples:
select 'Y' from dual;
select (5 * 3) cal_col from dual;
select (select min(col1) from table 2) calc_col from dual;
select nvl(col1, 'N') has_value from mytable;
From my experience, if you want to use a derived column in a select query, then you must define the column as part of an inner select. Here is an example:
select *
from (
select (col1 * col2) calc_col
from mytable
) data
where data.calc_col > 30
Another alternative is use the calculation within the where clause itself:
select (col1 * col2) calc_col
from mytable t
where (col1 * col2) > 30
If you are performing a count(*) operation, then you can also leverage the HAVING clause:
select field1, count(*)
from mytable
having count(*) > 3
I want to select distinct results from another select statement results
e.g;
select distinct from(select * from table)
following is result of inner select
testing department 9998901036 GOLD
testing department 9998901036 GOLD
I want to get distinct from above select result.
From your example, you could just do
select distinct * from table
But say you had some scenario where you wanted to distinct on some other results set, you could do
select distinct column1, column2 from (select * from table) T
Note that you have to alias your inner select
select distinct *
from
(select * from table) t
Works - You just need to give your sub select a table alias.
You can also use a CTE.
;WITH t AS
(
SELECT *
FROM table
)
SELECT DISTINCT *
FROM t