It seems we can use a SQL statement as:
select
(
select
count(*) as c_foos
from
foos
),
(
select
count(*) as c_bars
from
bars
);
but we can't do
select
(
select
count(*) as c_foos
from
foos
),
(
select
count(*) as c_bars
from
bars
),
(
select
(c_foos / c_bars) as the_ratio
);
or
select
(
select
count(*) as c_foos
from
foos
),
(
select
count(*) as c_bars
from
bars
),
(c_foos / c_bars) as the_ratio;
Is there a way to do that showing all 3 numbers? Is there a more definite rule as to what can be done and what can't?
You can try this:
You define two CTEs in a WITH clause, so you can use your result in the main query built on two cte tables (cte_num and cte_den)
WITH recursive
cte_num AS (
SELECT count(*) as c_foos
FROM foos
),
cte_den AS (
SELECT count(*) as c_bars
FROM bars
)
SELECT
cte_num.foos,
cte_den.bars,
cte_num.foos / cte_den.bars as the_ratio
from cte_num, cte_den;
There is a small number of simple rules... but SQL seems so easy that most programmers prefer to cut to the chase, and later complain they didn't get the plot :)
You can think of a query as a description of a flow: columns in a select share inputs (defined in from), but are evaluated "in parallel", without seeing each other. Your complex example boils down to the fact, that you cannot do this:
select 1 as a, 2 as b, a + b;
fields a and b are defined as outputs from the query, but there are no inputs called a and b. All you have to do is modify the query so that a and b are inputs:
select a + b from (select 1 as a, 2 as b) as inputs
And this will work (this is, btw., the solution for your queries).
Addendum:
The confusion comes from the fact, that in most SQL 101 cases outputs are created directly from inputs (data just passes through).
This flow model is useful, because it makes things easier to reason about in more complex cases. Also, we avoid ambiguities and loops. You can think about it in the context of query like: select name as last_name, last_name as name, name || ' ' || last_name from person;
Move the conditions to the FROM clause:
select f.c_foos, b.c_bars, f.c_foos / f.c_bars
from (select count(*) as c_foos from foos
) f cross join
(select count(*) as c_bars from bars
) b;
Ironically, your first version will work in MySQL (see here). I don't actually think this is intentional. I think it is an artifact of their parser -- meaning that it happens to work but might stop working in future versions.
The simplest way is to use a CTE that returns the 2 columns:
with cte as (
select
(select count(*) from foos) as c_foos,
(select count(*) from bars) as c_bars
)
select c_foos, c_bars, (c_foos / c_bars) as the_ratio
from cte
Note that the aliases of the 2 columns must be set outside of each query and not inside (the parentheses).
I have a problem with PL/SQL since i am new in PL/SQL world.
Let's say i have table like this.
COlumnA COlumnB COlumnC
1 5000000000 X
1 5000000000 X
2 4350000000 X
2 4350000000 X
3 10000000000 X
3 10000000000 X
3 10000000000 X
4 1809469720 Y
5 10000000000 X
5 10000000000 X
6 3000000000 X
6 3000000000 X
And i want to produce select statement as below.
ColumnC |Sum
X |32350000000
Y |1809469720
I have solved this problem in Oracle 12c with inner query, but when the system need to go to Oracle 11g, my query doesn't work anymore, i need to have the expected result with only one select statement.
Could anyone please advise?
Thank you!
This is what I came up with... using an inline view rather than a correlated subquery in the SELECT list.
SELECT d.columnc AS "ColumnC"
, SUM(d.columnb) AS "Sum"
FROM ( SELECT t.columna
, t.columnb
, t.columnc
FROM tablea t
GROUP
BY t.columna
, t.columnb
, t.columnc
) d
GROUP
BY d.columnc
This uses an inline view (aliased as "d") to return a "distinct" set of rows from tablea. We can get a distinct set, using a GROUP BY clause, or including the DISTINCT keyword, or even by writing a query that uses a UNION set operator.
Just wrap that query in parens, assign an alias, and use it in the FROM clause, as if it were a table or view.
The statement operates similarly to referencing a VIEW in the FROM clause.
You don't need to do this, but to illustrate how the query above operates. We could create a view, like this:
CREATE VIEW d AS
SELECT t.columna
, t.columnb
, t.columnc
FROM tablea t
GROUP
BY t.columna
, t.columnb
, t.columnc
And then we can reference the view in the FROM clause of another query, for example
SELECT d.columnc AS "ColumnC"
, SUM(d.columnb) AS "Sum"
FROM d
GROUP
BY d.columnc
But we don't actually need to create the VIEW object. We can include the view query as an "inline view".
I don't believe that Oracle 11g has a restriction on the nesting of inline views to three levels. I suspect that the restriction you are running into is related to correlated subqueries. The subquery can reference columns from the outer query, but only up one level... columns from the query it's used in. It can't reference columns in a query that is further out. (I've not confirmed with testing, but that's my recollection.)
This is where the actual ORA- and/or PLS- error message from Oracle would be of some help in identifying the restriction you are running into.
First find the distinct values of COlumnA,COlumnB and COlumnC then do the aggregation
Try this
select COlumnC,sum(COlumnB) from
(
select distinct COlumnA,COlumnB,COlumnC
from Table1
)
Group by COlumnC
Or you can simple use this query.
Select sum(columnB) as sum,columnC from table_name group by ColumnC;
Currently I have UNION's between 3 different select statements giving me three rows. What I'm needing is to modify this to have the union inside the from clause so that I can generate more columns (if I understand the functionality).
Basically what is going on is that the existing framework is designed and built to have a single row of data returned to it and will require heavy modification to handle a multi-row result set (everything is getting parsed to xml before being passed to the front-end).
My biggest issue (I believe) is being able to differentiate between the three sub selects inside the primary.
Guarantees in the select
1) Each select inside the from will only produce a single row result set.
2) All result sets from inside of the from will have same column count and column names (inherent of union I believe)
For example...
I currently have
SELECT * FROM A
UNION
SELECT * FROM B
UNION
SELECT * FROM C
Doing it this way produces a three row result set.
What I'm wanting if possible is....
SELECT cost as CurrentSelectedCost, /* from first select */
cost as PreviousCost, /* from second select */
cost as NextCost /* from third select */
FROM (
SELECT * FROM A
UNION
SELECT * FROM B
UNION
SELECT * FROM C
)
Now I'm guessing that I will need to alias the different select statements that are within the from clause, but I'm having issues getting that to function. The examples that I've found on here didn't seem to address the need to have all select statements inside of from differentiated. If this has been answered on here a link will suffice no reason to re-invent the wheel (I may just not know the terminology to search for) Also the database is a DB2 instance running on an iSeries
You need to unify the columns retrieved from the three SELECT statements.
SELECT CurrentSelectedCost,
PreviousCost,
NextCost
FROM (
SELECT cost as CurrentSelectedCost, 0 as PreviousCost, 0 as NextCost FROM A
UNION
SELECT 0 as CurrentSelectedCost, cost as PreviousCost, 0 as NextCost FROM B
UNION
SELECT 0 as CurrentSelectedCost, 0 as PreviousCost, cost as NextCost FROM C
) as COSTS
SELECT (SELECT COLNAME FROM A) CurrentSelectedCost,
(SELECT COLNAME FROM B) PreviousCost,
(SELECT COLNAME FROM C) NextCost
FROM DUAL
i dont think you need a union. there does not appear to be any join condition so maybe this will do it:
SELECT A.cost as CurrentSelectedCost, /* from first select */
B.cost as PreviousCost, /* from second select */
C.cost as NextCost /* from third select */
FROM A,B,C
I have a simple SQL query in PostgreSQL 8.3 that grabs a bunch of comments. I provide a sorted list of values to the IN construct in the WHERE clause:
SELECT * FROM comments WHERE (comments.id IN (1,3,2,4));
This returns comments in an arbitrary order which in my happens to be ids like 1,2,3,4.
I want the resulting rows sorted like the list in the IN construct: (1,3,2,4).
How to achieve that?
You can do it quite easily with (introduced in PostgreSQL 8.2) VALUES (), ().
Syntax will be like this:
select c.*
from comments c
join (
values
(1,1),
(3,2),
(2,3),
(4,4)
) as x (id, ordering) on c.id = x.id
order by x.ordering
In Postgres 9.4 or later, this is simplest and fastest:
SELECT c.*
FROM comments c
JOIN unnest('{1,3,2,4}'::int[]) WITH ORDINALITY t(id, ord) USING (id)
ORDER BY t.ord;
WITH ORDINALITY was introduced with in Postgres 9.4.
No need for a subquery, we can use the set-returning function like a table directly. (A.k.a. "table-function".)
A string literal to hand in the array instead of an ARRAY constructor may be easier to implement with some clients.
For convenience (optionally), copy the column name we are joining to ("id" in the example), so we can join with a short USING clause to only get a single instance of the join column in the result.
Works with any input type. If your key column is of type text, provide something like '{foo,bar,baz}'::text[].
Detailed explanation:
PostgreSQL unnest() with element number
Just because it is so difficult to find and it has to be spread: in mySQL this can be done much simpler, but I don't know if it works in other SQL.
SELECT * FROM `comments`
WHERE `comments`.`id` IN ('12','5','3','17')
ORDER BY FIELD(`comments`.`id`,'12','5','3','17')
With Postgres 9.4 this can be done a bit shorter:
select c.*
from comments c
join (
select *
from unnest(array[43,47,42]) with ordinality
) as x (id, ordering) on c.id = x.id
order by x.ordering;
Or a bit more compact without a derived table:
select c.*
from comments c
join unnest(array[43,47,42]) with ordinality as x (id, ordering)
on c.id = x.id
order by x.ordering
Removing the need to manually assign/maintain a position to each value.
With Postgres 9.6 this can be done using array_position():
with x (id_list) as (
values (array[42,48,43])
)
select c.*
from comments c, x
where id = any (x.id_list)
order by array_position(x.id_list, c.id);
The CTE is used so that the list of values only needs to be specified once. If that is not important this can also be written as:
select c.*
from comments c
where id in (42,48,43)
order by array_position(array[42,48,43], c.id);
I think this way is better :
SELECT * FROM "comments" WHERE ("comments"."id" IN (1,3,2,4))
ORDER BY id=1 DESC, id=3 DESC, id=2 DESC, id=4 DESC
Another way to do it in Postgres would be to use the idx function.
SELECT *
FROM comments
ORDER BY idx(array[1,3,2,4], comments.id)
Don't forget to create the idx function first, as described here: http://wiki.postgresql.org/wiki/Array_Index
In Postgresql:
select *
from comments
where id in (1,3,2,4)
order by position(id::text in '1,3,2,4')
On researching this some more I found this solution:
SELECT * FROM "comments" WHERE ("comments"."id" IN (1,3,2,4))
ORDER BY CASE "comments"."id"
WHEN 1 THEN 1
WHEN 3 THEN 2
WHEN 2 THEN 3
WHEN 4 THEN 4
END
However this seems rather verbose and might have performance issues with large datasets.
Can anyone comment on these issues?
To do this, I think you should probably have an additional "ORDER" table which defines the mapping of IDs to order (effectively doing what your response to your own question said), which you can then use as an additional column on your select which you can then sort on.
In that way, you explicitly describe the ordering you desire in the database, where it should be.
sans SEQUENCE, works only on 8.4:
select * from comments c
join
(
select id, row_number() over() as id_sorter
from (select unnest(ARRAY[1,3,2,4]) as id) as y
) x on x.id = c.id
order by x.id_sorter
SELECT * FROM "comments" JOIN (
SELECT 1 as "id",1 as "order" UNION ALL
SELECT 3,2 UNION ALL SELECT 2,3 UNION ALL SELECT 4,4
) j ON "comments"."id" = j."id" ORDER BY j.ORDER
or if you prefer evil over good:
SELECT * FROM "comments" WHERE ("comments"."id" IN (1,3,2,4))
ORDER BY POSITION(','+"comments"."id"+',' IN ',1,3,2,4,')
And here's another solution that works and uses a constant table (http://www.postgresql.org/docs/8.3/interactive/sql-values.html):
SELECT * FROM comments AS c,
(VALUES (1,1),(3,2),(2,3),(4,4) ) AS t (ord_id,ord)
WHERE (c.id IN (1,3,2,4)) AND (c.id = t.ord_id)
ORDER BY ord
But again I'm not sure that this is performant.
I've got a bunch of answers now. Can I get some voting and comments so I know which is the winner!
Thanks All :-)
create sequence serial start 1;
select * from comments c
join (select unnest(ARRAY[1,3,2,4]) as id, nextval('serial') as id_sorter) x
on x.id = c.id
order by x.id_sorter;
drop sequence serial;
[EDIT]
unnest is not yet built-in in 8.3, but you can create one yourself(the beauty of any*):
create function unnest(anyarray) returns setof anyelement
language sql as
$$
select $1[i] from generate_series(array_lower($1,1),array_upper($1,1)) i;
$$;
that function can work in any type:
select unnest(array['John','Paul','George','Ringo']) as beatle
select unnest(array[1,3,2,4]) as id
Slight improvement over the version that uses a sequence I think:
CREATE OR REPLACE FUNCTION in_sort(anyarray, out id anyelement, out ordinal int)
LANGUAGE SQL AS
$$
SELECT $1[i], i FROM generate_series(array_lower($1,1),array_upper($1,1)) i;
$$;
SELECT
*
FROM
comments c
INNER JOIN (SELECT * FROM in_sort(ARRAY[1,3,2,4])) AS in_sort
USING (id)
ORDER BY in_sort.ordinal;
select * from comments where comments.id in
(select unnest(ids) from bbs where id=19795)
order by array_position((select ids from bbs where id=19795),comments.id)
here, [bbs] is the main table that has a field called ids,
and, ids is the array that store the comments.id .
passed in postgresql 9.6
Lets get a visual impression about what was already said. For example you have a table with some tasks:
SELECT a.id,a.status,a.description FROM minicloud_tasks as a ORDER BY random();
id | status | description
----+------------+------------------
4 | processing | work on postgres
6 | deleted | need some rest
3 | pending | garden party
5 | completed | work on html
And you want to order the list of tasks by its status.
The status is a list of string values:
(processing, pending, completed, deleted)
The trick is to give each status value an interger and order the list numerical:
SELECT a.id,a.status,a.description FROM minicloud_tasks AS a
JOIN (
VALUES ('processing', 1), ('pending', 2), ('completed', 3), ('deleted', 4)
) AS b (status, id) ON (a.status = b.status)
ORDER BY b.id ASC;
Which leads to:
id | status | description
----+------------+------------------
4 | processing | work on postgres
3 | pending | garden party
5 | completed | work on html
6 | deleted | need some rest
Credit #user80168
I agree with all other posters that say "don't do that" or "SQL isn't good at that". If you want to sort by some facet of comments then add another integer column to one of your tables to hold your sort criteria and sort by that value. eg "ORDER BY comments.sort DESC " If you want to sort these in a different order every time then... SQL won't be for you in this case.