How to use UPDATE in PostgreSQL with variable table? - sql

Question

Example:
Table=["category1","category2","category3"]
for varTable in Table:
cr.execute('update varTable SET id=%s,WHERE id=%s)
........
....
How to do this loop?

Answer 1

Use dynamic SQL for that. The default is to use plpgsql with EXECUTE.
Create a function or use a DO statement for ad-hoc execution.
Dynamic SQL
CREATE OR REPLACE FUNCTION f_up(_new_id int, _old_id int)
RETURNS void AS
$BODY$
DECLARE
_tbl text[] := '{category1,category2,category3}';
t text;
BEGIN
FOREACH t IN ARRAY _tbl
LOOP
EXECUTE '
UPDATE ' || t || '
SET id = $1
WHERE id = $2'
USING _new_id, _old_id;
END LOOP;
END;
$BODY$ LANGUAGE plpgsql;
Call:
SELECT f_up(23, 45);
There are lots of similar answers on SO. Search for dynamic-sql, plpgsql and EXECUTE for more examples and explanation.
Plain SQL
If plpgsql is still black magic to you, you can solve this simple case quite effectively with a data-modifying CTE. Requires PostgreSQL 9.1 - for data-modifying CTE.
WITH vals AS (
SELECT 23 AS new_id, 45 AS old_id -- provide values once
)
, a AS (
UPDATE category1
SET id = v.new_id
FROM vals v
WHERE id = v.old_id
)
, b AS (
UPDATE category2
SET id = v.new_id
FROM vals v
WHERE id = v.old_id
)
UPDATE category3
SET id = v.new_id
FROM vals v
WHERE id = v.old_id;