如何使用 select、最大日期并在 sql 查询中声明？

Question

I am trying to use declare and max in variable. This is the query below:

Declare @MAX_BUF Datetime
 
Set @OpeningStock = (
                    SELECT @MAX_BUF = MAX(end_date) FROM [IBIS].[buf_stk]
                    WHERE SUBSTRING(CONVERT(VARCHAR,end_date ,112 ),1,6)<SUBSTRING(CONVERT(VARCHAR,getdate() ,112 ),1,6);
                    
                    SELECT COUNT(1) AS Opening_Stock 
                    FROM [IBIS].[buf_stk] AS bs(NOLOCK)
                    WHERE CAST(end_date AS DATE)=@MAX_BUF
                    )

I am getting syntax error in '=' and '(end_date)'. Please let me know if this can be resolved.

Answer 1

Without seeing your table and data I can't be confident in rewriting your SQL correctly, however all you need to do to "make it work", I suspect, is amend slightly to:

SELECT @MAX_BUF = MAX(end_date) FROM [IBIS].[buf_stk]
                    WHERE SUBSTRING(CONVERT(VARCHAR,end_date ,112 ),1,6)<SUBSTRING(CONVERT(VARCHAR,getdate() ,112 ),1,6);
                    
                    SELECT @OpeningStock=COUNT(*)
                    FROM [IBIS].[buf_stk] AS bs(NOLOCK) --<< Remove this!
                    WHERE CAST(end_date AS DATE)=@MAX_BUF

Having said that, you could combine these into a single query. Your manipulation of dates as strings is not sargable and will force the optimizer to scan your table/index.

If you want to find rows where end_date is prior to 1st day of the month then just compare as dates (assuming your end_date is an actual date data type)

where end_date < DATEADD(month, DATEDIFF(month, 0, GetDate()), 0)

And lose the nolock hint, unless you prefer your data to be randomly incorrect.

Answer 2

Use two statements. I'm not a fan of converting dates to strings for date arithmetic. You seem to want the maximum date from before this month. So, DATEDIFF() provides one method (and there are more efficient methods if you have an index on end_date ):

Set @@MAX_BUF = (SELECT CAST(MAX(end_date) as date)
                 FROM [IBIS].[buf_stk]
                 WHERE DATEDIFF(month, end_date, getdate()) >= 1
                );

Set @OpeningStock = (SELECT COUNT(1) AS Opening_Stock 
                     FROM [IBIS].[buf_stk] bs
                     WHERE CAST(end_date AS DATE) = @MAX_BUF
                    );

You can also do this as a single statement:

select top (1)
       @max_buf = CAST(end_date as date),
       @OpeningStock = COUNT(*)
from FROM [IBIS].[buf_stk] bs
where end_date < dateadd(day, 1 - day(getdate()), convert(date, getdate()))
group by CAST(end_date as date)
order by CAST(end_date as date) desc;

Note that this also changes the date comparison to be friendlier to the optimizer.

Answer 3

You can actually do this all in one statement and one scan of the table:

SELECT @MAX_BUF = MAX(end_date), @OpeningStock = COUNT(1)
FROM
    (SELECT TOP (1) WITH TIES
        CAST(end_date AS date) end_date
    FROM [IBIS].[buf_stk]
    WHERE end_date >= DATEADD(month, 1, getdate())
    ORDER BY CAST(end_date AS date) DESC
    ) t;

Notes:

• Don't use NOLOCK , it has many unintended effects and can cause incorrect results
• Switch round the WHERE predicate in order to hit an index if you have one. Don't make the server do algebra for you, it's not very good at it.