结果查询为空时 case 返回 null

Question

I try to initialize variable from table, but when result of query returns 0 row case expression is not working, for example:

do $$
declare _reportId bigint := null;
        _serviceId bigint := null; 
begin

    _serviceId := 12;
    _reportId := null;

    select  case when _serviceId is null then rp.value::bigint else _serviceId end
    into    _serviceId
    from    rep.reportparameters as rp
    where   rp.reportid = _reportId;

    RAISE NOTICE '%', _serviceId;
end;
$$; 

So in this variant of parameters i need _serviceId value 12 (because result of query returns 0 row, is no reportId with null in table). But i have null value for _serviceId

Analog construction in SQL Server work great, they return 12 for @serviceId :

DECLARE @reportId bigint = NULL, 
        @serviceId bigint = null; 

SET @serviceId = 12;

SELECT  CASE when @serviceId is null then rp.[value] else @serviceId end
from    rep.reportparameters as rp
where   rp.reportid = @reportId;

SELECT @serviceId 

Why in PostgreSql case not working?

Answer 1

As documented in the manual , if no row is returned, then the INTO assignment will set the variable to NULL. The CASE expression is never evaluated if no row is selected, so it can't change a NULL value into something different.

You can use coalesce() with a direct assignment instead:

do $$
declare 
  _reportId bigint := null;
  _serviceId bigint := null; 
begin

    _reportId := null;
    _serviceid := 12;
    _serviceid := coalesce((select rp.value::bigint
                            from rep.reportparameters as rp
                            where rp.reportid = _reportId), _serviceid);

    RAISE NOTICE '%', _serviceId;
end;
$$;