Best practices for asserting some condition in SQL when creating a table? - sql

Question

Imagine I create some table:
CREATE TABLE mytable AS
...
Now I want to conduct some sanity check, verify some condition is true for every record of mytable. I could frame this problem as checking whether the result of another query returns zero results.
SELECT count(*)
FROM mytable
WHERE something_horrible_is_true
Is there a standard, recommended way to generate an error here if the count is not equal to zero? To make something happen such that if I'm executing this sanity check query using a java.sql.Statement, a SQLException is triggered?
Is this a reasonable approach? Or is this a better way to enforce that some condition is always true when creating a table? (I use Postgresql.)

Answer 1

Create function to raise exception:
create or replace function raise_error(text) returns varchar as $body$
begin
raise exception '%', $1;
return null;
end; $body$ language plpgsql volatile;
Then you can use it in a regular SQLs:
SELECT case when count(*) > 0 then raise_error('something horrible is true!') end
FROM mytable
WHERE something_horrible_is_true
Here you will get the SQL exception if there are rows that satisfy the something_horrible_is_true condition.
There are also several more complex usage examples:
SELECT
case
when count(*) = 0 then raise_error('something horrible is true!')::int
else count(*)
end
FROM mytable
WHERE something_horrible_is_true
Returns count or rise exception when nothing found.
update mytable set
mydatefield =
case
when mydatefield = current_date then raise_error('Can not update today''s rows')::date
else '1812-10-10'::date
end;
Prevents to update some rows (this is a somewhat contrived example but it shows yet another usage way)
... and so on.

Answer 2

Are you familiar with triggers? Postresql provides good suport for triggers especially using the pgsql laguange.
A trigger is a function (check) that is always run on an event: insert, update,delete. You can call the function before or after the event.
I believe once you know this concept, you can find an online tutorial to help you achieve your goal.
A general approach may look like this:
CREATE FUNCTION trigger_function() RETURN trigger AS
$$
DECLARE c integer;
BEGIN
SELECT count(*) into c FROM mytable WHERE something_horrible_is_true;
IF c>0 then RAISE EXCEPTION 'cannot have a negative salary';
END IF;
return new;
END;
$$ LANGUAGE plpgsql;
And afterwards you execute
CREATE TRIGGER trigger_name BEFORE INSERT
ON table_name
FOR EACH ROW
EXECUTE PROCEDURE trigger_function()
Both code sections are pqsql.