PL/SQL：在单个查询中选择多个变量

Question

在 oracle 包的初始化部分，我尝试确定具有给定名称的某些对象的当前 ID。原因：该包用于多个数据库，这些对象的 ID 可能会有所不同，但它们的名称是不变的。

代码如下所示：

SELECT id INTO varCat FROM myTable WHERE Type = 'Object' AND Name = 'Cat';
SELECT id INTO varDog FROM myTable WHERE Type = 'Object' AND Name = 'Dog';
...
SELECT id INTO varMouse FROM myTable WHERE Type = 'Object' AND Name = 'Mouse';

有没有办法优化查询，也许可以在单个查询中完成？

Answer 1

您可以将它们与手动枢轴结合使用：

select max(case when name = 'Cat' then id end),
  max(case when name = 'Dog' then id end),
  max(case when name = 'Mouse' then id end)
into varCat, varDog, varMouse
from mytable
where type = 'Object'
and name in ('Cat', 'Dog', 'Mouse');

快速演示：

create table mytable (id number, type varchar2(10), name varchar2(10));
insert into mytable (id, type, name) values (1, 'Object', 'Mouse');
insert into mytable (id, type, name) values (2, 'Object', 'Cat');
insert into mytable (id, type, name) values (3, 'Object', 'Dog');

set serveroutput on
declare
  varCat mytable.id%type;
  varDog mytable.id%type;
  varMouse mytable.id%type;
begin
  select max(case when name = 'Cat' then id end),
    max(case when name = 'Dog' then id end),
    max(case when name = 'Mouse' then id end)
  into varCat, varDog, varMouse
  from mytable
  where type = 'Object'
  and name in ('Cat', 'Dog', 'Mouse');

  dbms_output.put_line('varCat: ' || varCat);
  dbms_output.put_line('varDog: ' || varDog);
  dbms_output.put_line('varMouse: ' || varMouse);
end;
/

varCat: 2
varDog: 3
varMouse: 1

PL/SQL procedure successfully completed.

如果类型和名称的组合不唯一，那么您当前的代码会出错（行太多）；这将无声地选择最高的 ID。

如果您还获得名称可能相同的其他类型的 ID，您也可以在 case 表达式中包含该类型：

select max(case when type = 'Object' and name = 'Cat' then id end),
  max(case when type = 'Object' and name = 'Dog' then id end),
  max(case when type = 'Object' and name = 'Mouse' then id end)
  -- , ... other combinations you want to get
into varCat, varDog, varMouse --, ... other variables
from mytable
where (type = 'Object' and name in ('Cat', 'Dog', 'Mouse'))
or ... ;