SQL Server将表联接到自身而不是在子查询中作为选择的效率

Question

This might sound like a dumb question - apologies if this has been answered before (almost certainly) but I couldn't find an answer with search.

I've got a query that is aggregating values in a table as a subquery in different ways for different columns, eg for a transaction on a given day, transactions in the previous month, previous 6 months, before that, after that. I aliased the main table as tx, then the subquery alias as tx1

SELECT 
    tr.transaction_value
    ,ISNULL(
         (SELECT SUM(tr1.transaction_value) FROM [MyDB].[dbo].[Transactions] tr1
         WHERE tr1.Client_Ref = tr.Client_Ref),0) AND tr1.Transaction_Date > tr.Transaction_Date 
     ),0) as 'Future_Transactions'
    ,ISNULL(
         (SELECT SUM(tr1.transaction_value) FROM [MyDB].[dbo].[Transactions] tr1
         WHERE tr1.Client_Ref = tr.Client_Ref),0) AND tr1.Transaction_Date < tr.Transaction_Date 
     ),0) as 'Prior_Transactions'
FROM [MyDB].[dbo].[Transactions] tr 

I want to know, if I have 7 such columns with subqueries, defining the table 7 times in the subqueries, is it possible and better (more efficient, more readable) to create tx1 as an INNER JOIN on the main table query rather than recreating it in each subquery, and if so how would I code the WHERE clauses? Thanks :o)

Answer 1

This should be equivalent and better performing.

SELECT 
    tr.transaction_value
    ,SUM(tr1.transaction_value) as 'Future_Transactions'
    ,SUM(tr2.transaction_value) as 'Prior_Transactions'
FROM [MyDB].[dbo].[Transactions] tr
LEFT JOIN [MyDB].[dbo].[Transactions] tr1 ON tr1.Client_Ref = tr.Client_Ref AND tr1.Transaction_Date > tr.Transaction_Date 
LEFT JOIN [MyDB].[dbo].[Transactions] tr2 ON tr2.Client_Ref = tr.Client_Ref AND tr2.Transaction_Date < tr.Transaction_Date 
group by tr.transaction_value