Re: why isn't this subquery wrong?

[Steve Crawford <scrawford@xxxxxxxxxxxxxxxxxxxx>]

On Thu, Apr 20, 2017 at 3:17 PM, jonathan vanasco <postgres@xxxxxxxx> wrote:

I ran into an issue while changing a database schema around.  Some queries still worked, even though I didn't expect them to.

Can anyone explain to me why the following is valid (running 9.6) ?

schema

CREATE TEMPORARY TABLE example_a__data (

foo_id INT,

bar_id INT

);

CREATE TEMPORARY TABLE example_a__rollup_source (

id int primary key,

name varchar(64),

foo_id INT,

check_bool BOOLEAN

);

CREATE TEMPORARY TABLE example_a__rollup AS

SELECT id, name, foo_id

FROM example_a__rollup_source

WHERE check_bool IS TRUE

;

query:

SELECT foo_id

FROM example_a__data

WHERE foo_id IN (SELECT bar_id FROM example_a__rollup)

;

a raw select of `SELECT bar_id FROM example_a__rollup;` will cause an error because bar_id doesn't exist

postgres doesn't raise an error because example_a__data does have a bar_id -- but example_a__rollup doesn't and there's no explicit correlation in the query.

can someone explain why this happens?  i'm guessing there is a good reason -- but I'm unfamiliar with the type of implicit join/queries this behavior is enabling.

There is no requirement in this query that bar_id be in the example_a__rollup table and since it is only in one table it is unambiguous so the server doesn't complain.

It may be explanatory to add a couple records to your example_a_rollup table:

insert into example_a__data values (3,4),(5,6);

Then run a simple select showing what the where clause would see:

SELECT,

   foo_id, 
   (SELECT bar_id FROM example_a__rollup) 
FROM

    example_a__data 
;

 foo_id | bar_id                                                                                           --------+-------- 

     3 | 

     5 |      

bar_id is null because there are no rows in example_a_rollup.

Now add a single record to example_a_rollup:

insert into example_a__rollup (id) values (10);

Rerun the query and you will get:

 foo_id | bar_id  
--------+--------
     3 |      4
     5 |      6

If you add another record to example_a__rollup and run it and you will get:

ERROR:  more than one row returned by a subquery used as an _expression_

Although the subquery won't work as an _expression_ it would still work in a the where clause but I doubt it will return what you desire. Unfortunately there are lots of ways to write syntactically correct but logically flawed statements.

Cheers,

Steve