Left join with dynamic table name derived from column - sql

Question

I am new in PostgreSQL and I wonder if it's possible to use number from table tbc as part of the table name in left join 'pa' || number. So for example if number is 456887 I want left join with table pa456887. Something like this:
SELECT tdc.cpa, substring(tdc.ku,'[0-9]+') AS number, paTab.vym
FROM public."table_data_C" AS tdc
LEFT JOIN concat('pa' || number) AS paTab ON (paTab.cpa = tdc.cpa)
And I want to use only PostgreSQL, not additional code in PHP for example.

Answer 1

Either way, you need dynamic SQL.
Table name as given parameter
CREATE OR REPLACE FUNCTION foo(_number int)
RETURNS TABLE (cpa int, nr text, vym text) AS -- adapt to actual data types!
$func$
BEGIN
RETURN QUERY EXECUTE format(
'SELECT t.cpa, substring(t.ku,'[0-9]+'), p.vym
FROM public."table_data_C" t
LEFT JOIN %s p USING (cpa)'
, 'pa' || _number
);
END
$func$ LANGUAGE plpgsql;
Call:
SELECT * FROM foo(456887)
Generally, you would sanitize table names with format ( %I ) to avoid SQL injection. With just an integer as dynamic input that's not necessary. More details and links in this related answer:
INSERT with dynamic table name in trigger function
Data model
There may be good reasons for the data model. Like partitioning / sharding or separate privileges ...
If you don't have such a good reason, consider consolidating multiple tables with identical schema into one and add the number as column. Then you don't need dynamic SQL.
Consider inheritance. Then you can add a condition on tableoid to only retrieve rows from a given child table:
SELECT * FROM parent_table
WHERE tableoid = 'pa456887'::regclass
Be aware of limitations for inheritance, though. Related answers:
Get the name of a row's source table when querying the parent it inherits from
Select (retrieve) all records from multiple schemas using Postgres
Name of 2nd table depending on value in 1st table
Deriving the name of the join table from values in the first table dynamically complicates things.
For only a few tables
LEFT JOIN each on tableoid. There is only one match per row, so use COALESCE.
SELECT t.*, t.tbl, COALESCE(p1.vym, p2.vym, p3.vym) AS vym
FROM (
SELECT cpa, ('pa' || substring(ku,'[0-9]+'))::regclass AS tbl
FROM public."table_data_C"
-- WHERE <some condition>
) t
LEFT JOIN pa456887 p1 ON p1.cpa = t.cpa AND p1.tableoid = t.tbl
LEFT JOIN pa456888 p2 ON p2.cpa = t.cpa AND p2.tableoid = t.tbl
LEFT JOIN pa456889 p3 ON p3.cpa = t.cpa AND p3.tableoid = t.tbl
For many tables
Combine a loop with dynamic queries:
CREATE OR REPLACE FUNCTION foo(_number int)
RETURNS TABLE (cpa int, nr text, vym text) AS
$func$
DECLARE
_nr text;
BEGIN
FOR _nr IN
SELECT DISTINCT substring(ku,'[0-9]+')
FROM public."table_data_C"
LOOP
RETURN QUERY EXECUTE format(
'SELECT t.cpa, _nr, p.vym
FROM public."table_data_C" t
LEFT JOIN %I p USING (cpa)
WHERE t.ku LIKE (_nr || '%')'
, 'pa' || _nr
);
END LOOP;
END
$func$ LANGUAGE plpgsql;