I want to select a column from a table, with the column name being the result of a query like the following:
-- This query returns a single value
with x as (
select a from table1 where <condition>
)
-- my_function() yields a table
select x from my_function()
How do I do that?
Thank you very much.
You could write it in SQL with a temporary function:
CREATE FUNCTION pg_temp.tablefunc()
RETURNS SETOF my_function_result_type
LANGUAGE plpgsql AS
$$DECLARE
v_colname text;
BEGIN
SELECT a INTO v_colname
FROM table1
LIMIT 1;
RETURN QUERY EXECUTE
format(E'SELECT %I\n'
'FROM my_function()',
v_colname);
END;$$;
SELECT * FROM pg_temp.tablefunc();
Related
I have the function in which I have prepared dynamic query, which I want print in output window before executing it.
Note: In the following example I have just add simple select statement to understand the requirement.
Sample tables:
create table t1
(
col1 int,
col2 text
);
insert into t1 values(1,'Table T1');
insert into t1 values(2,'Table T1');
create table t2
(
col1 int,
col2 text
);
insert into t2 values(1,'Table T2');
insert into t2 values(2,'Table T2');
Function:
create or replace function fn_testing(tbl_Name text)
returns table(col1 int,col2 text) as
$$
begin
return query execute 'select col1,col2 from '||tbl_name||'';
end;
$$
language plpgsql;
Function call:
select * from fn_testing('t2');
I want to print following in message window with result set too in result window:
select col1,col2 from t1;
You can use RAISE NOTICE for messages.
CREATE OR REPLACE FUNCTION fn_testing
(_tbl_name name)
RETURNS TABLE
(col1 integer,
col2 text)
AS
$$
DECLARE
_query text;
BEGIN
_query := format('SELECT col1, col2 FROM %I;', _tbl_name);
RAISE NOTICE '%', _query;
RETURN QUERY EXECUTE _query;
END;
$$
LANGUAGE plpgsql;
Note: There's a special type, name, for identifiers. And to prevent SQL injection or errors you should make sure the dynamic identifiers are properly quoted. You can use format() with %I for that.
This stored function returns a query:
DROP FUNCTION IF EXISTS get_query (
ctl text, scm text, tbl text, seq text);
CREATE OR REPLACE FUNCTION get_query (
ctl text, scm text, tbl text, seq text)
RETURNS text
AS
$$
select concat('insert into ',$2,'.',$1, ' select nextval("',$4,'") as id, ',
string_agg(concat('NEW.', column_name), ', '), ', current_timestamp as audited_at;')
from information_schema.columns
where table_catalog = $1
and table_schema = $2
and table_name = $3
$$
LANGUAGE sql;
How do I PREPARE the query that this function returns.
I want insert a record in a table when a trigger is fired but I don't want to specify the list of columns to be inserted. The schema might keep changing. Hence, trying to use prepared statements.
This sample code illustrates how I mean the query string to be executed:
DROP FUNCTION IF EXISTS fn_name (store_temporary_query text);
CREATE OR REPLACE FUNCTION fn_name (store_temporary_query text)
RETURNS table (query text)
LANGUAGE plpgsql
AS
$$
begin
select 'select 1 as ID' into store_temporary_query;
return query (select store_temporary_query);
end;
$$
select fn_name('');
The above query gives the following output
fn_name
select 1 as ID
The desired result is the query
ID
1
EDIT #2
DROP FUNCTION IF EXISTS fn_name (store_temporary_query text);
CREATE OR REPLACE FUNCTION fn_name (store_temporary_query text)
RETURNS table (query text)
LANGUAGE plpgsql
AS
$$
begin
select 'select 1 as ID;' into store_temporary_query;
return query execute store_temporary_query;
end;
$$
select fn_name('');
This gets us here,
Error executing SQL statement. ERROR: syntax error at or near "select"
Position: 254 - Connection: Aurora Legacy: 794ms
You need EXECUTE to execute a query stored in a string:
RETURN QUERY EXECUTE store_temporary_query;
I'm trying to use CTE in PostgreSQL function and returning the CTE as table. But I couldn't manage to compile the function as it says ERROR: syntax error at end of input in the select query. Could someone point me what I'm missing here.
CREATE OR REPLACE FUNCTION my_func(name varchar) RETURNS TABLE (hours integer) AS $$
BEGIN
WITH a AS (
SELECT hours FROM name_table tbl where tbl.name= name; <- giving error here
)
RETURN QUERY SELECT hours FROM a;
END;
$$ LANGUAGE plpgsql;
PS: I'm on PostgreSQL 9.6 if that helps.
The CTE expression is part of the query, so it needs to come immediately after the return query clause, not before it. Additionally, to avoid syntax errors later on, you should select a parameter name that ins't ambiguous with the names of the columns, and fully qualify the columns you're querying:
CREATE OR REPLACE FUNCTION my_func(v_name varchar)
RETURNS TABLE (hours integer) AS $$
BEGIN
RETURN QUERY WITH a AS (
SELECT tbl.hours
FROM name_table tbl
WHERE name = v_name
)
SELECT a.hours FROM a;
END;
$$ LANGUAGE plpgsql;
I have multiple tables with each two rows of interest: connection_node_start_id and connection_node_end_id. My goal is to get a collection of all those IDs, either as a flat ARRAY or as a new TABLE consisting of one row.
Example output ARRAY:
result = {1,4,7,9,2,5}
Example output TABLE:
IDS
-------
1
4
7
9
2
5
My fist attempt is somewhat clumsy and does not work properly as the SELECT statement just returns one row. It seems there must be a simple way to do this, can someone point me into the right direction?
CREATE OR REPLACE FUNCTION get_connection_nodes(anyarray)
RETURNS anyarray AS
$$
DECLARE
table_name varchar;
result integer[];
sel integer[];
BEGIN
FOREACH table_name IN ARRAY $1
LOOP
RAISE NOTICE 'table_name(%)',table_name;
EXECUTE 'SELECT ARRAY[connection_node_end_id,
connection_node_start_id] FROM ' || table_name INTO sel;
RAISE NOTICE 'sel(%)',sel;
result := array_cat(result, sel);
END LOOP;
RETURN result;
END
$$
LANGUAGE 'plpgsql';
Test table:
connection_node_start_id | connection_node_end_id
--------------------------------------------------
1 | 4
7 | 9
Call:
SELECT get_connection_nodes(ARRAY['test_table']);
Result:
{1,4} -- only 1st row, rest is missing
For Postgres 9.3+
CREATE OR REPLACE FUNCTION get_connection_nodes(text[])
RETURNS TABLE (ids int) AS
$func$
DECLARE
_tbl text;
BEGIN
FOREACH _tbl IN ARRAY $1
LOOP
RETURN QUERY EXECUTE format('
SELECT t.id
FROM %I, LATERAL (VALUES (connection_node_start_id)
, (connection_node_end_id)) t(id)'
, _tbl);
END LOOP;
END
$func$ LANGUAGE plpgsql;
Related answer on dba.SE:
SELECT DISTINCT on multiple columns
Or drop the loop and concatenate a single query. Probably fastest:
CREATE OR REPLACE FUNCTION get_connection_nodes2(text[])
RETURNS TABLE (ids int) AS
$func$
BEGIN
RETURN QUERY EXECUTE (
SELECT string_agg(format(
'SELECT t.id FROM %I, LATERAL (VALUES (connection_node_start_id)
, (connection_node_end_id)) t(id)'
, tbl), ' UNION ALL ')
FROM unnest($1) tbl
);
END
$func$ LANGUAGE plpgsql;
Related:
Loop through like tables in a schema
LATERAL was introduced with Postgres 9.3.
For older Postgres
You can use the set-returning function unnest() in the SELECT list, too:
CREATE OR REPLACE FUNCTION get_connection_nodes2(text[])
RETURNS TABLE (ids int) AS
$func$
BEGIN
RETURN QUERY EXECUTE (
SELECT string_agg(
'SELECT unnest(ARRAY[connection_node_start_id
, connection_node_end_id]) FROM ' || tbl
, ' UNION ALL '
)
FROM (SELECT quote_ident(tbl) AS tbl FROM unnest($1) tbl) t
);
END
$func$ LANGUAGE plpgsql;
Should work with pg 8.4+ (or maybe even older). Works with current Postgres (9.4) as well, but LATERAL is much cleaner.
Or make it very simple:
CREATE OR REPLACE FUNCTION get_connection_nodes3(text[])
RETURNS TABLE (ids int) AS
$func$
BEGIN
RETURN QUERY EXECUTE (
SELECT string_agg(format(
'SELECT connection_node_start_id FROM %1$I
UNION ALL
SELECT connection_node_end_id FROM %1$I'
, tbl), ' UNION ALL ')
FROM unnest($1) tbl
);
END
$func$ LANGUAGE plpgsql;
format() was introduced with pg 9.1.
Might be a bit slower with big tables because each table is scanned once for every column (so 2 times here). Sort order in the result is different, too - but that does not seem to matter for you.
Be sure to sanitize escape identifiers to defend against SQL injection and other illegal syntax. Details:
Table name as a PostgreSQL function parameter
The EXECUTE ... INTO statement can only return data from a single row:
If multiple rows are returned, only the first will be assigned to the INTO variable.
In order to concatenate values from all rows you have to aggregate them first by column and then append the arrays:
EXECUTE 'SELECT array_agg(connection_node_end_id) ||
array_agg(connection_node_start_id) FROM ' || table_name INTO sel;
You're probably looking for something like this:
CREATE OR REPLACE FUNCTION d (tblname TEXT [])
RETURNS TABLE (c INTEGER) AS $$
DECLARE sql TEXT;
BEGIN
WITH x
AS (SELECT unnest(tblname) AS tbl),
y AS (
SELECT FORMAT('
SELECT connection_node_end_id
FROM %s
UNION ALL
SELECT connection_node_start_id
FROM %s
', tbl, tbl) AS s
FROM x)
SELECT string_agg(s, ' UNION ALL ')
INTO sql
FROM y;
RETURN QUERY EXECUTE sql;
END;$$
LANGUAGE plpgsql;
CREATE TABLE a (connection_node_end_id INTEGER, connection_node_start_id INTEGER);
INSERT INTO A VALUES (1,2);
CREATE TABLE b (connection_node_end_id INTEGER, connection_node_start_id INTEGER);
INSERT INTO B VALUES (100, 101);
SELECT * from d(array['a','b']);
c
-----
1
2
100
101
(4 rows)
Can we able to use SELECT statement within CASE conditional expression in function?
I have tried the following function to accomplish the above task.
Example: I have a function which is used to display table containing some rows.
create or replace function test(n integer)
returns table (name text,city text) as
$body$
begin
case n when 1 then
select * from table1
when 2 then
select * from table2
when 3 then
select * from view1
end;
end;
$body$
language plpgsql;
--Calling function
select * from test(1);
/*have to show details of table1*/
select * from test(2);
/*have to show details of table2*/
select * from test(3);
/*have to display details of view1*/
Actually, there is a plpgsql CASE statement, not to be confused with the SQL CASE expression:
CREATE OR REPLACE function test(n integer)
RETURNS TABLE (name text, city text) AS
$func$
BEGIN
CASE n
WHEN 1 THEN
RETURN QUERY SELECT t.name, t.city FROM table1 t;
WHEN 2 THEN
RETURN QUERY SELECT t.foo, t.bar FROM table2 t;
WHEN 3 THEN
RETURN QUERY SELECT t.bar, t.bamm FROM view1 t;
END CASE;
END
$func$ LANGUAGE plpgsql;
If you declare your function as RETURNS TABLE (name text, city text), then your SELECT statements should have a column list with matching types.
If on the other hand you want to SELECT *, declare the function as RETURNS SETOF table1 accordingly.
When naming columns in the return type, those variable are visible in the function body. Be sure to table-qualify column names that would conflict with same names. So t.name instead of just name.
Column names from queries inside the function are not visible outside. Only the declared return type. So names don't have to match, just data types.
Either way, I suggest to simplify things:
CREATE OR REPLACE function test(n integer)
RETURNS SETOF table1 AS
$func$
SELECT * FROM table1 t WHERE n = 1
UNION ALL
SELECT * FROM table2 t WHERE n = 2
UNION ALL
SELECT * FROM view1 t WHERE n = 3;
$func$ LANGUAGE sql;
Same result. Just as fast. SQL or PL/pgSQL function is a matter of taste and some other details. PL/pgSQL is probably faster for stand-alone calls. SQL can more easily be nested.
For this you need an IF statement and RETURN QUERY, e.g.
create or replace function test(n integer)
returns table (name text,city text) as
$body$
begin
IF n = 1 THEN
RETURN QUERY select * from table1;
ELIF n = 2 THEN
RETURN QUERY select * from table2;
ELIF n = 3 THEN
RETURN QUERY select * from view1;
END IF;
end;
$body$
language plpgsql;
It's a very weird thing to want to do, though.