I have a table with n columns and I need to create a view which contains the frequencies of every unique value in every column. n is unknown since, I need to apply the solution on numerous tables with different number of columns.
For example i have table:
column1 column2 column3
value1 value2 value3
value2 value2 value1
value1 value2 value2
The view should be something like this:
columnname value frequency
column1 value1 2
column1 value2 1
column2 value2 3
...
Since I have very little experience with sql any help would be extremely appreciated.
Many thanks in advance!
Thus far I have come up with this but am sort of stonewalled now.
CREATE or REPLACE FUNCTION create_view () RETURNS setof record AS $$
DECLARE
col RECORD;
BEGIN
for col in execute 'select column_name from information_schema.columns
where table_name = ''table123''' LOOP
???
END LOOP;
return;
END;
$$
LANGUAGE 'plpgsql';
Simple SQL:
SELECT 'col1' AS col, col1 AS val, count(*) AS ct FROM tbl GROUP BY col1
UNION ALL
SELECT 'col2', col2, count(*) FROM tbl GROUP BY col2
UNION ALL
SELECT 'col3', col3, count(*) FROM tbl GROUP BY col3
PL/pqSQL function executing dynamic SQL:
CREATE OR REPLACE FUNCTION f_demo(_schema text, _tbl text)
RETURNS TABLE(col text, val text, ct bigint) AS
$xx$
DECLARE
_fld text;
_sql text := '';
BEGIN
FOR _fld IN
SELECT a.attname -- use quote_ident to safeguard against SQLi
FROM pg_catalog.pg_attribute a
WHERE a.attrelid = (COALESCE(_schema || '.', '') || _tbl)::regclass
AND a.attnum > 0
AND NOT a.attisdropped
-- AND a.attname ~~ '%col%' -- if you want to pick specific columns
LOOP
RETURN QUERY EXECUTE
'SELECT $1, ' || quote_ident(_fld) || '::text, count(*)
FROM ' || COALESCE(quote_ident(_schema) || '.', '') || quote_ident(_tbl) || '
GROUP BY 2'
USING _fld;
END LOOP;
END;
$xx$
LANGUAGE 'plpgsql';
Call:
SELECT * FROM f_demo('public', 'mytable');
Or, if you want to use the schema provided by search_path:
SELECT * FROM f_demo(NULL, 'mytable');
Major points
Works for any table and any number of columns with values of any type.
Values are cast to text to simplify my example. Could be done with a polymorphic type, too.
See this related answer for info on plpgsql techniques and links: https://stackoverflow.com/q/8146245/939860
Related
In PgSQL I make huge select, and then I want count it's size and apply some extra filters.
execute it twice sound dumm,
so I wrapped it in function
and then "cache" it and return union of filtered table and extra row at the end where in "id" column store size
with q as (select * from myFunc())
select * from q
where q.distance < 400
union all
select count(*) as id, null,null,null
from q
but it also doesn't look like proper solution...
and so the question: is in pg something like "generator function" or any other stuff that can properly solve this ?
postgreSQL 13
myFunc aka "selectItemsByRootTag"
CREATE OR REPLACE FUNCTION selectItemsByRootTag(
in tag_name VARCHAR(50)
)
RETURNS table(
id BIGINT,
name VARCHAR(50),
description TEXT,
/*info JSON,*/
distance INTEGER
)
AS $$
BEGIN
RETURN QUERY(
WITH RECURSIVE prod AS (
SELECT
tags.name, tags.id, tags.parent_tags
FROM
tags
WHERE tags.name = (tags_name)
UNION
SELECT c.name, c.id , c.parent_tags
FROM
tags as c
INNER JOIN prod as p
ON c.parent_tags = p.id
)
SELECT
points.id,
points.name,
points.description,
/*points.info,*/
points.distance
from points
left join tags on points.tag_id = tags.id
where tags.name in (select prod.name from prod)
);
END;
$$ LANGUAGE plpgsql;
as a result i want see maybe set of 2 table or generator function that yield some intermediate result not shure how exacltly it should look
demo
CREATE OR REPLACE FUNCTION pg_temp.selectitemsbyroottag(tag_name text, _distance numeric)
RETURNS TABLE(id bigint, name text, description text, distance numeric, count bigint)
LANGUAGE plpgsql
AS $function$
DECLARE _sql text;
BEGIN
_sql := $p1$WITH RECURSIVE prod AS (
SELECT
tags.name, tags.id, tags.parent_tags
FROM
tags
WHERE tags.name ilike '%$p1$ || tag_name || $p2$%'
UNION
SELECT c.name, c.id , c.parent_tags
FROM
tags as c
INNER JOIN prod as p
ON c.parent_tags = p.id
)
SELECT
points.id,
points.name,
points.description,
points.distance,
count(*) over ()
from points
left join tags on points.tag_id = tags.id
where tags.name in (select prod.name from prod)
and points.distance > $p2$ || _distance
;
raise notice '_sql: %', _sql;
return query execute _sql;
END;
$function$
You can call it throug following way
select * from pg_temp.selectItemsByRootTag('test',20);
select * from pg_temp.selectItemsByRootTag('test_8',20) with ORDINALITY;
The 1 way to call the function, will have a row of total count total number of rows. Second way call have number of rows plus a serial incremental number.
I also make where q.distance < 400 into function input argument.
selectItemsByRootTag('test',20); means that q.distance > 20 and tags.name ilike '%test%'.
I have a very big table but as an example I will only provide a very small part of it as following:-
col1 col2 col3 col4
10 2 12
13 4 11
0 1
3 5 111
I know how to find null values in one column. What I want to find is how many null values are there in each column just by writing one query.
Thanks in advance
You can use an aggregate with a filter:
select count(*) filter (where col1 is null) as col1_nulls,
count(*) filter (where col2 is null) as col2_nulls,
count(*) filter (where col3 is null) as col3_nulls,
count(*) filter (where col4 is null) as col4_nulls
from the_table;
I think you can generate this query on the fly. Here is an example of one way you can approach it:
CREATE OR REPLACE FUNCTION null_counts(tablename text)
RETURNS SETOF jsonb LANGUAGE plpgsql AS
$func$
BEGIN
RETURN QUERY EXECUTE 'SELECT to_jsonb(t) FROM (SELECT ' || (
SELECT string_agg('count(*) filter (where ' || a.attname::text || ' is null) as ' || a.attname || '_nulls', ',')
FROM pg_catalog.pg_attribute a
WHERE a.attrelid = tablename::regclass
AND a.attnum > 0
AND a.attisdropped = false
) || ' FROM ' || tablename::regclass || ') as t';
END
$func$;
SELECT null_counts('your_table') AS val;
I am trying to write sub-queries so that I search all tables for a column named id and since there are multiple tables with id column, I want to add the condition, so that id = 3119093.
My attempt was:
Select *
from information_schema.tables
where id = '3119093' and id IN (
Select table_name
from information_schema.columns
where column_name = 'id' );
This didn't work so I tried:
Select *
from information_schema.tables
where table_name IN (
Select table_name
from information_schema.columns
where column_name = 'id' and 'id' IN (
Select * from table_name where 'id' = 3119093));
This isn't the right way either. Any help would be appreciated. Thanks!
A harder attempt is:
CREATE OR REPLACE FUNCTION search_columns(
needle text,
haystack_tables name[] default '{}',
haystack_schema name[] default '{public}'
)
RETURNS table(schemaname text, tablename text, columnname text, rowctid text)
AS $$
begin
FOR schemaname,tablename,columnname IN
SELECT c.table_schema,c.table_name,c.column_name
FROM information_schema.columns c
JOIN information_schema.tables t ON
(t.table_name=c.table_name AND t.table_schema=c.table_schema)
WHERE (c.table_name=ANY(haystack_tables) OR haystack_tables='{}')
AND c.table_schema=ANY(haystack_schema)
AND t.table_type='BASE TABLE'
--AND c.column_name = "id"
LOOP
EXECUTE format('SELECT ctid FROM %I.%I WHERE cast(%I as text) like %L',
schemaname,
tablename,
columnname,
needle
) INTO rowctid;
IF rowctid is not null THEN
RETURN NEXT;
END IF;
END LOOP;
END;
$$ language plpgsql;
select * from search_columns('%3119093%'::varchar,'{}'::name[]) ;
The only problem is this code displays the table name and column name. I have to then manually enter
Select * from table_name where id = 3119093
where I got the table name from the code above.
I want to automatically implement returning rows from a table but I don't know how to get the table name automatically.
I took the time to make it work for you.
For starters, some information on what is going on inside the code.
Explanation
function takes two input arguments: column name and column value
it requires a created type that it will be returning a set of
first loop identifies tables that have a column name specified as the input argument
then it forms a query which aggregates all rows that match the input condition inside every table taken from step 3 with comparison based on ILIKE - as per your example
function goes into the second loop only if there is at least one row in currently visited table that matches specified condition (then the array is not null)
second loop unnests the array of rows that match the condition and for every element it puts it in the function output with RETURN NEXT rec clause
Notes
Searching with LIKE is inefficient - I suggest adding another input argument "column type" and restrict it in the lookup by adding a join to pg_catalog.pg_type table.
The second loop is there so that if more than 1 row is found for a particular table, then every row gets returned.
If you are looking for something else, like you need key-value pairs, not just the values, then you need to extend the function. You could for example build json format from rows.
Now, to the code.
Test case
CREATE TABLE tbl1 (col1 int, id int); -- does contain values
CREATE TABLE tbl2 (col1 int, col2 int); -- doesn't contain column "id"
CREATE TABLE tbl3 (id int, col5 int); -- doesn't contain values
INSERT INTO tbl1 (col1, id)
VALUES (1, 5), (1, 33), (1, 25);
Table stores data:
postgres=# select * From tbl1;
col1 | id
------+----
1 | 5
1 | 33
1 | 25
(3 rows)
Creating type
CREATE TYPE sometype AS ( schemaname text, tablename text, colname text, entirerow text );
Function code
CREATE OR REPLACE FUNCTION search_tables_for_column (
v_column_name text
, v_column_value text
)
RETURNS SETOF sometype
LANGUAGE plpgsql
STABLE
AS
$$
DECLARE
rec sometype%rowtype;
v_row_array text[];
rec2 record;
arr_el text;
BEGIN
FOR rec IN
SELECT
nam.nspname AS schemaname
, cls.relname AS tablename
, att.attname AS colname
, null::text AS entirerow
FROM
pg_attribute att
JOIN pg_class cls ON att.attrelid = cls.oid
JOIN pg_namespace nam ON cls.relnamespace = nam.oid
WHERE
cls.relkind = 'r'
AND att.attname = v_column_name
LOOP
EXECUTE format('SELECT ARRAY_AGG(row(tablename.*)::text) FROM %I.%I AS tablename WHERE %I::text ILIKE %s',
rec.schemaname, rec.tablename, rec.colname, quote_literal(concat('%',v_column_value,'%'))) INTO v_row_array;
IF v_row_array is not null THEN
FOR rec2 IN
SELECT unnest(v_row_array) AS one_row
LOOP
rec.entirerow := rec2.one_row;
RETURN NEXT rec;
END LOOP;
END IF;
END LOOP;
END
$$;
Exemplary call & output
postgres=# select * from search_tables_for_column('id','5');
schemaname | tablename | colname | entirerow
------------+-----------+---------+-----------
public | tbl1 | id | (1,5)
public | tbl1 | id | (1,25)
(2 rows)
table1
id col1 col2 col3...
table2
col_id col_name
3432 col1
5342 col2
6756 col3
Now I want to generate table 3 like this:
id col_name col_value col_id
Please note that col1, col2,col3... are not in order. Therefore I have to query table2 to obtain col_id ( I think pivot does not work here)
How can I do it in SQL?
It appears that you want a select like this:
SELECT t2.id,
CASE
WHEN t2.col_name='col1' THEN t1.col1
WHEN t2.col_name='col2' THEN t1.col2
WHEN t2.col_name='col2' THEN t1.col2
-- ... more columns
ELSE NULL
END
FROM table2 t2 LEFT JOIN table2 t1 ON t2.col_id = t1.id
You could also create a function, though this will be slower in practice:
CREATE OR REPLACE FUNCTION table1_col(id integer, name text) RETURNS text as $$
DECLARE
col_val text;
BEGIN
EXECUTE format('SELECT %s FROM table1 WHERE id=$1', name)
INTO col_val
USING id;
RETURN col_val;
END;
$$ LANGUAGE plpgsql;
SELECT table1_col(col_id,col_name) FROM table2;
In Postgres 8.4 or higher, what is the most efficient way to get a row of data populated by defaults without actually creating the row. Eg, as a transaction (pseudocode):
create table "mytable"
(
id serial PRIMARY KEY NOT NULL,
parent_id integer NOT NULL DEFAULT 1,
random_id integer NOT NULL DEFAULT random(),
)
begin transaction
fake_row = insert into mytable (id) values (0) returning *;
delete from mytable where id=0;
return fake_row;
end transaction
Basically I'd expect a query with a single row where parent_id is 1 and random_id is a random number (or other function return value) but I don't want this record to persist in the table or impact on the primary key sequence serial_id_seq.
My options seem to be using a transaction like above or creating views which are copies of the table with the fake row added but I don't know all the pros and cons of each or whether a better way exists.
I'm looking for an answer that assumes no prior knowledge of the datatypes or default values of any column except id or the number or ordering of the columns. Only the table name will be known and that a record with id 0 should not exist in the table.
In the past I created the fake record 0 as a permanent record but I've come to consider this record a type of pollution (since I typically have to filter it out of future queries).
You can copy the table definition and defaults to the temp table with:
CREATE TEMP TABLE table_name_rt (LIKE table_name INCLUDING DEFAULTS);
And use this temp table to generate dummy rows. Such table will be dropped at the end of the session (or transaction) and will only be visible to current session.
You can query the catalog and build a dynamic query
Say we have this table:
create table test10(
id serial primary key,
first_name varchar( 100 ),
last_name varchar( 100 ) default 'Tom',
age int not null default 38,
salary float default 100.22
);
When you run following query:
SELECT string_agg( txt, ' ' order by id )
FROM (
select 1 id, 'SELECT ' txt
union all
select 2, -9999 || ' as id '
union all
select 3, ', '
|| coalesce( column_default, 'null'||'::'||c.data_type )
|| ' as ' || c.column_name
from information_schema.columns c
where table_schema = 'public'
and table_name = 'test10'
and ordinal_position > 1
) xx
;
you will get this sting as a result:
"SELECT -9999 as id , null::character varying as first_name ,
'Tom'::character varying as last_name , 38 as age , 100.22 as salary"
then execute this query and you will get the "phantom row".
We can build a function that build and excecutes the query and return our row as a result:
CREATE OR REPLACE FUNCTION get_phantom_rec (p_i test10.id%type )
returns test10 as $$
DECLARE
v_sql text;
myrow test10%rowtype;
begin
SELECT string_agg( txt, ' ' order by id )
INTO v_sql
FROM (
select 1 id, 'SELECT ' txt
union all
select 2, p_i || ' as id '
union all
select 3, ', '
|| coalesce( column_default, 'null'||'::'||c.data_type )
|| ' as ' || c.column_name
from information_schema.columns c
where table_schema = 'public'
and table_name = 'test10'
and ordinal_position > 1
) xx
;
EXECUTE v_sql INTO myrow;
RETURN myrow;
END$$ LANGUAGE plpgsql ;
and then this simple query gives you what you want:
select * from get_phantom_rec ( -9999 );
id | first_name | last_name | age | salary
-------+------------+-----------+-----+--------
-9999 | | Tom | 38 | 100.22
I would just select the fake values as literals:
select 1 id, 1 parent_id, 1 user_id
The returned row will be (virtually) indistinguishable from a real row.
To get the values from the catalog:
select
0 as id, -- special case for serial type, just return 0
(select column_default::int -- Cast to int, because we know the column is int
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'mytable'
and column_name = 'parent_id') as parent_id,
(select column_default::int -- Cast to int, because we know the column is int
from INFORMATION_SCHEMA.COLUMNS
where table_name = 'mytable'
and column_name = 'user_id') as user_id;
Note that you must know what the columns are and their type, but this is reasonable. If you change the table schema (except default value), you would need to tweak the query.
See the above as a SQLFiddle.