Re: Moving from TSQL to PL/pgsql select into a variable...

Hi

po 18. 3. 2019 v 15:53 odesílatel Ron Clarke <rclarkeai(at)gmail(dot)com> napsal:

> /*
> I'm trying to get to grips with the world of pgsql and have lots of habits
> and techniques from a lifetime of SQL Server's TSQL.
>
> Anyway - I'm trying to assign a variable with the value returned by a
> select statement to be used subsequently within a pl/pgsql code block..
> (in real life I want to maniupulate json objects)
>

you cannot to do it. There is lot of differences between PL/pgSQL and T-SQL.

a) DO statements doesn't support returning value

b) T-SQL procedures allows returns multirecord set as result of unbound
queries. Nothing similar is in PL/pgSQL (nad PL/SQL (Oracle) or DB2).

> To keep this absolutely simple as an example let's assign the value 'a' to
> my variable from a select so :
>
> SELECT cast('a' as varchar) AS X;
>
> Then if I do the following:
>
> */
>
> DO $$
> DECLARE myvar varchar;
> BEGIN
> BEGIN
> -- Try SELECT 'a' INTO myvar;
> SELECT cast('a' as varchar) AS X INTO myvar;
>

don't do it, use assign statement - it is faster and cleaner

myvar := cast('a' as varchar);

>
> EXCEPTION
> WHEN NO_DATA_FOUND THEN
> myvar := NULL;
> END;
> SELECT myvar;
>

returning is not supported in this case. In other cases, there is RETURN
statement

END;
> $$
> LANGUAGE plpgsql ;
>
> /*
>
> ERROR: query has no destination for result data
> SQL state: 42601
> Hint: If you want to discard the results of a SELECT, use PERFORM instead.
> Context: PL/pgSQL function inline_code_block line 13 at SQL statement
>
> */
>
> DO $$
> DECLARE myvar varchar;
> BEGIN
> BEGIN
> -- SELECT cast('a' as varchar) AS X INTO myvar;
> PERFORM cast('a' as varchar) AS X INTO myvar;
>
> EXCEPTION
> WHEN NO_DATA_FOUND THEN
> myvar := NULL;
> END;
> SELECT myvar;
> END;
> $$
> LANGUAGE plpgsql ;
>
> /*
> ERROR: query "SELECT cast('a' as varchar) AS X INTO myvar" is not a SELECT
> SQL state: 42601
> Context: PL/pgSQL function inline_code_block line 7 at PERFORM
> */
>
> /*
>
> I'm guessing that I'm using the wrong approach here and fighting pqsql as
> opposed to using it, so can anyone point me in the right direction as to
> what I should be doing to select and assign a value to a variable as I've
> managed to confuse myself now.
>

It is little bit joke, but all what you know from T-SQL is better to
forgot, if you want to write code in PLpgSQL. Please, try to read doc. It
is good enough

https://www.postgresql.org/docs/9.6/plpgsql.html

Regards

Pavel

>
> Thanks
> */
>
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