数据库左外部连接到具有最佳性能的Same表

Question

I have a status table A with Code & its description related to a particular name.

Code   Desc       Name
 01    INITIAL   STATUS_A
 02    SUCCESS   STATUS_A
 03    FAILED    STATUS_A
 04    FAILED    STATUS_B
 05    RETRY     STATUS_C
 06    SUCESS    STATUS_D

and so on... Now I have a table B as master record with code associate from Table A.

ID  column1 column2 column3 statusA statusB statusC statusD 
432   XXXX   YYYY    ZZZZ      03     04      05      06      

Now I need to form a query to get output data with code description from table A.

I have done this using left outer join. Can someone suggest a better solution.

Left Outer Join Query

Select b.column1 ,b.column2, a1.Desc as Status_A_Desc , a2.Desc as Status_B_Desc, a3.Desc as Status_C_Desc, a4.Desc as Status_D_Desc from B b left outer join A a1 on a1.code = b.statusA and a1.Name ='STATUS_A' left outer join A a2 on a2.code = b.statusB and a2.Name ='STATUS_B' left outer join A a3 on a3.code = b.statusC and a3.Name ='STATUS_C' left outer join A a4 on a4.code = b.statusD and a4.Name ='STATUS_D' where b.ID = 432

Answer 1

You can use conditional aggregation instead:

select a.code,
       max(case when a.name = 'Status_A' then a.desc end) as a,
       max(case when a.name = 'Status_B' then a.desc end) as b,
       max(case when a.name = 'Status_C' then a.desc end) as c,
       max(case when a.name = 'Status_D' then a.desc end) as d
from a
group by a.code;

Depending on your database, the joins might be better. This has the advantage that you can add as many fields as you like with basically the same performance.