I'm using the Entity-Attribute-Value (EAV) pattern to store 'overrides' for target objects. That is, there are three tables:
Entity, contains the target records
Attribute, contains the column names of 'overridable' columns in the Entity table
Override, contains the EAV records
What I'd like to do is select Overrides along with the value of the 'overridden' column from the Entity table. Thus, requiring dynamic use of the Attribute name in the SQL.
My naive attempt in (PostgreSQL) SQL:
SELECT
OV.entity_id as entity,
AT.name as attribute,
OV.value as value,
ENT.base_value as base_value
FROM "override" AS OV
LEFT JOIN "attribute" as AT
ON (OV.attribute_id = AT.id)
LEFT JOIN LATERAL (
SELECT
id,
AT.name as base_value -- AT.name doesn't resolve to a SQL identifier
FROM "entity"
) AS ENT
ON ENT.id = OV.entity_id;
This doesn't work as AT.name doesn't resolve to a SQL identifier and simply returns column names such as 'col1', 'col2', etc. rather than querying Entity with the column name.
I'm aware this is dynamic SQL but I'm pretty new to PL/pgSQL and couldn't figure out as it is correlated/lateral joined. Plus, is this even possible since the column types are not homogeneously typed? Note all the 'values' in the Override table are stored as strings to get round this problem.
Any help would be most appreciated!
You can use PL/pgSQL to dynamically request the columns. I'm assuming the following simplified database structure (all original and overide values are "character varying" in this example as I didn't find any further type information):
CREATE TABLE public.entity (
id integer NOT NULL DEFAULT nextval('entity_id_seq'::regclass),
attr1 character varying,
attr2 character varying,
<...>
CONSTRAINT entity_pkey PRIMARY KEY (id)
)
CREATE TABLE public.attribute (
id integer NOT NULL DEFAULT nextval('attribute_id_seq'::regclass),
name character varying,
CONSTRAINT attribute_pkey PRIMARY KEY (id)
)
CREATE TABLE public.override (
entity_id integer NOT NULL,
attribute_id integer NOT NULL,
value character varying,
CONSTRAINT override_pkey PRIMARY KEY (entity_id, attribute_id),
CONSTRAINT override_attribute_id_fkey FOREIGN KEY (attribute_id)
REFERENCES public.attribute (id),
CONSTRAINT override_entity_id_fkey FOREIGN KEY (entity_id)
REFERENCES public.entity (id))
With the PL/pgSQL function
create or replace function get_base_value(
entity_id integer,
column_identifier character varying
)
returns setof character varying
language plpgsql as $$
declare
begin
return query execute 'SELECT "' || column_identifier || '" FROM "entity" WHERE "id" = ' || entity_id || ';';
end $$;
you can use almost exactly your query:
SELECT
OV.entity_id as entity,
AT.name as attribute,
OV.value as value,
ENT.get_base_value as base_value
FROM "override" AS OV
LEFT JOIN "attribute" as AT
ON (OV.attribute_id = AT.id)
LEFT JOIN LATERAL (
SELECT id, get_base_value FROM get_base_value(OV.entity_id, AT.name)
) AS ENT
ON ENT.id = OV.entity_id;
Related
I am trying to generate a column which is not stored in the database. The column obtains its value from a function. The problem is there is an error of 'no such function:'
What should I do to get the value from this function and assign it to the column?
My SQL script so far:
CREATE FUNCTION GETAVERAGE (#locationID AS INTEGER)
RETURNS NUMERIC
AS
BEGIN
DECLARE #avg AS NUMERIC
SET #avg = SELECT AVG(value) FROM properties WHERE location_id = #locationID
END;
CREATE TABLE IF NOT EXISTS properties
(
property_id INTEGER PRIMARY KEY,
location_id INTEGER,
address VARCHAR (100),
value NUMERIC,
average_prop_value NUMERIC GENERATED ALWAYS AS (GETAVERAGE(location_id)) VIRTUAL,
FOREIGN KEY (location_id) REFERENCES locations (location_id)
);
CREATE TABLE IF NOT EXISTS locations
(
location_id INTEGER PRIMARY KEY,
name VARCHAR (50) UNIQUE
);
Not sure which database you are using. If you add a specific tag (and version detail) to the question, that could help. It could be MySQL. Here's standard SQL that would work with most databases (of a recent version):
SELECT t.*
, AVG(value) OVER (PARTITION BY location_id) AS avg_value
FROM properties t
;
Use this in a VIEW, if you wish.
I have the following tables:
CREATE TABLE "User" (
id integer DEFAULT nextval('"User_id_seq"'::regclass) PRIMARY KEY,
name text NOT NULL DEFAULT ''::text,
coinflips boolean[]
);
CREATE TABLE "User_coinflips_COPY" (
"nodeId" integer,
position integer,
value boolean,
id integer DEFAULT nextval('"User_coinflips_COPY_id_seq"'::regclass) PRIMARY KEY
);
I'm no looking for the SQL statement that grabs the value entry from each row in User_coinflips and inserts it as an array into the coinflips column on User.
Any help would be appreciated!
Update
Not sure if it's important but I just realized a minor mistake in my table definitions above, I replace User_coinflips with User_coinflips_COPY since that accurately describes my schema. Just for context, before it looked like this:
CREATE TABLE "User_coinflips" (
"nodeId" integer REFERENCES "User"(id) ON DELETE CASCADE,
position integer,
value boolean NOT NULL,
CONSTRAINT "User_coinflips_pkey" PRIMARY KEY ("nodeId", position)
);
You are looking for an UPDATE, rather then insert.
Use a derived table with the aggregated values to join against in the UPDATE statement:
update "User"
set conflips = t.flips
from (
select "nodeId", array_agg(value order by position) as flips
from "User_coinflips"
group by "nodeId"
) t
where t."nodeId" = "User"."nodeId";
I want to check if the same two attribute values exist in two different tables. If the combination from table_a is not existing in table_b it should be inserted into the select statement table. Right now I have the following query, which is working:
CREATE TABLE table_a (
attr_a integer,
attr_b text,
uuid character varying(200),
CONSTRAINT table_a_pkey PRIMARY KEY (uuid)
);
CREATE TABLE table_b (
attr_a integer,
attr_b text,
uuid character varying(200),
CONSTRAINT table_b_pkey PRIMARY KEY (uuid)
);
SELECT * FROM table_a
WHERE (table_a.attr_a::text || table_a.attr_b::text) != ALL(SELECT (table_b.attr_a::text || table_b.attr_a::text) FROM table_b)
However, the execution time is pretty long. So I would like to ask if there is a faster solution to check for that.
Your where clause uses a manipulation of attr_a (casting it to text and concatinating with attr_b), so the index can't be used. Instead of this concatination, why not try a straight-forward exists operator?
SELECT *
FROM table_a a
WHERE NOT EXISTS (SELECT *
FROM table_b b
WHERE a.attr_a = b.attr_a AND
b.attr_b = b.attr_b)
I have two Postgres tables:
create table A(
id_A serial not null,
column_A varchar null;
...);
create table B(
id_B serial not null,
id_A int4 not null,
name varchar null,
keywords varchar null,
...);
An element of table A is associated to multiple elements of table B and an element of table B is associated to one element of table A.
The column keywords in table B is a concatenation of values of columns B.name and A.column_A:
B.keywords := B.name || A.column_A
How to update with a trigger the column B.keywords of each row in table B if the value of A.column_A is updated?
In other words, I want to do something like this (pseudo-code):
FOR EACH ROW current_row IN TABLE B
UPDATE B SET keywords = (SELECT B.name || A.column_A
FROM B INNER JOIN A ON B.id_A = A.id_A
WHERE B.id_B = current_row.id_B)
WHERE id_B = current_row.id_B;
Your trigger has to call a function when A is updated:
CREATE OR REPLACE FUNCTION update_b()
RETURNS TRIGGER
AS $$
BEGIN
UPDATE B
SET keywords = name || NEW.column_A
WHERE id_A = NEW.id_A;
return NEW;
END
$$ LANGUAGE plpgsql;
CREATE TRIGGER update_b_trigger AFTER UPDATE OF column_A
ON A
FOR EACH ROW
EXECUTE PROCEDURE update_b();
It might also be useful to add a trigger BEFORE INSERT OR UPDATE on table B to set the keywords.
Your approach is broken by design. Do not try to keep derived values current in the table. That's not safe for concurrent access. All kinds of complications can arise. You bloat table B (and backups) and impair write performance.
Instead, use a VIEW (or a MATERIALIZED VIEW):
CREATE VIEW ab AS
SELECT B.*, concat_ws(', ', B.name, A.column_A) AS keywords
FROM B
LEFT JOIN A USING (id_A);
With the updated table definition below referential integrity is guaranteed and you can use [INNER] JOIN instead of LEFT [OUTER] JOIN.
Or even a simple query might be enough ...
Either way, you need a PRIMARY KEY constraint in table A and a FOREIGN KEY constraint in table B:
CREATE TABLE A (
id_A serial PRIMARY KEY,
column_A varchar
...);
CREATE TABLE B (
id_B serial PRIMARY KEY,
id_A int4 NOT NULL REFERENCES A(id_A),
name varchar
-- and *no* redundant "keywords" column!
...);
About concatenating strings:
How to concatenate columns in a Postgres SELECT?
And I wouldn't use CaMeL-case identifiers:
Are PostgreSQL column names case-sensitive?
I have a view, and I have a function that returns records from this view.
Here is the view definition:
CREATE VIEW ctags(id, name, descr, freq) AS
SELECT tags.conc_id, expressions.name, concepts.descr, tags.freq
FROM tags, concepts, expressions
WHERE concepts.id = tags.conc_id
AND expressions.id = concepts.expr_id;
The column id references to the table tags, that, references to another table concepts, which, in turn, references to the table expressions.
Here are the table definitions:
CREATE TABLE expressions(
id serial PRIMARY KEY,
name text,
is_dropped bool DEFAULT FALSE,
rank float(53) DEFAULT 0,
state text DEFAULT 'never edited',
UNIQUE(name)
);
CREATE TABLE concepts(
id serial PRIMARY KEY,
expr_id int NOT NULL,
descr text NOT NULL,
source_id int,
equiv_p_id int,
equiv_r_id int,
equiv_len int,
weight int,
is_dropped bool DEFAULT FALSE,
FOREIGN KEY(expr_id) REFERENCES expressions,
FOREIGN KEY(source_id),
FOREIGN KEY(equiv_p_id) REFERENCES concepts,
FOREIGN KEY(equiv_r_id) REFERENCES concepts,
UNIQUE(id,equiv_p_id),
UNIQUE(id,equiv_r_id)
);
CREATE TABLE tags(
conc_id int NOT NULL,
freq int NOT NULL default 0,
UNIQUE(conc_id, freq)
);
The table expressions is also referenced from my view (ctags).
I want my function to combine rows of my view, that have equal values in the column name and that refer to rows of the table concepts with equal values of the column equiv_r_id so that these rows are combined only once, the combined row has one (doesn't matter which) of the ids, the value of the column descr is concatenated from the values of the rows being combined, and the row freq contains the sum of the values from the rows being combined. I have no idea how to do it, any help would be appreciated.
Basically, what you describe looks like this:
CREATE FUNCTION f_test()
RETURNS TABLE(min_id int, name text, all_descr text, sum_freq int) AS
$x$
SELECT min(t.conc_id) -- AS min_id
,e.name
,string_agg(c.descr, ', ') -- AS all_descr
,sum(t.freq) -- AS sum_freq
FROM tags t
JOIN concepts c USING (id)
JOIN expressions e ON e.id = c.expr_id;
-- WHERE e.name IS DISTINCT FROM
$x$
LANGUAGE sql;
Major points:
I ignored the view ctags altogether as it is not needed.
You could also write this as View so far, the function wrapper is not necessary.
You need PostgreSQL 9.0+ for string_agg(). Else you have to substitute with
array_to_string(array_agg(c.descr), ', ')
The only unclear part is this:
and that refer to rows of the table concepts with equal values of the column equiv_r_id so that these rows are combined only once
Waht column exactly refers to what column in table concepts?
concepts.equiv_r_id equals what exactly?
If you can clarify that part, I might be able to incorporate it into the solution.