如何减少很多相似的相关子查询？

Question

这是一个更大的声明的一部分，但我想知道 CTE 或其他方法是否有助于使其更高效或更清晰。我可以将其编写为表值函数并将其包含在我的 from 子句中，但如果有其他解决方案，我想避免额外的对象。

当我的费率的生效日期早于基表时，这里的 SELECT TOP 1 ... 子查询只是捕获，但我不喜欢为我需要的每一列重复它们使用权。有没有更好的方法来实现这一点，或者这是一个正常的声明？

SELECT j.EmployeeId
       ,j.CompanyId
       ,j.JobCode
       ,COALESCE(j.CustomWageRate, r.WageRate, (SELECT TOP 1 WageRate
                                                FROM   ContractLabor.CompanyJobRates
                                                WHERE  CompanyId = j.CompanyId
                                                       AND JobCode = j.JobCode
                                                       AND EffectiveDate < j.EffectiveDate
                                                ORDER  BY EffectiveDate DESC), 0) AS EffectiveRate
       ,COALESCE(r.CustomBurdenRateReg, (SELECT TOP 1 CustomBurdenRateReg
                                         FROM   ContractLabor.CompanyJobRates
                                         WHERE  CompanyId = j.CompanyId
                                                AND JobCode = j.JobCode
                                                AND EffectiveDate < j.EffectiveDate
                                         ORDER  BY EffectiveDate DESC)) AS CustomBurdenRateReg
       ,COALESCE(r.CustomBurdenRateOvt, (SELECT TOP 1 CustomBurdenRateOvt
                                         FROM   ContractLabor.CompanyJobRates
                                         WHERE  CompanyId = j.CompanyId
                                                AND JobCode = j.JobCode
                                                AND EffectiveDate < j.EffectiveDate
                                         ORDER  BY EffectiveDate DESC)) AS CustomBurdenRateOvt
       ,COALESCE(r.CustomBurdenRateDbl, (SELECT TOP 1 CustomBurdenRateDbl
                                         FROM   ContractLabor.CompanyJobRates
                                         WHERE  CompanyId = j.CompanyId
                                                AND JobCode = j.JobCode
                                                AND EffectiveDate < j.EffectiveDate
                                         ORDER  BY EffectiveDate DESC)) AS CustomBurdenRateDbl
       ,j.EffectiveDate
FROM   ContractLabor.EmployeeJobDetails j
       LEFT JOIN ContractLabor.CompanyJobRates r
         ON j.CompanyId = r.CompanyId
            AND j.JobCode = r.JobCode
            AND j.EffectiveDate = r.EffectiveDate

Answer 1

SELECT j.EmployeeId
       ,j.CompanyId
       ,j.JobCode
       ,COALESCE(j.CustomWageRate, r.WageRate, ca.WageRate, 0) AS EffectiveRate
       ,COALESCE(r.CustomBurdenRateReg, ca.CustomBurdenRateReg) AS CustomBurdenRateReg
       ,COALESCE(r.CustomBurdenRateOvt, ca.CustomBurdenRateOvt) AS CustomBurdenRateOvt
       ,COALESCE(r.CustomBurdenRateDbl, ca.CustomBurdenRateDbl) AS CustomBurdenRateDbl
       ,j.EffectiveDate
FROM   ContractLabor.EmployeeJobDetails j
       LEFT JOIN ContractLabor.CompanyJobRates r
         ON j.CompanyId = r.CompanyId
            AND j.JobCode = r.JobCode
            AND j.EffectiveDate = r.EffectiveDate

       OUTER APPLY --or CROSS APPLY
       (
            SELECT TOP 1 WageRate
                    ,CustomBurdenRateReg
                    ,CustomBurdenRateOvt
                    ,CustomBurdenRateDbl
            FROM   ContractLabor.CompanyJobRates
            WHERE  CompanyId = j.CompanyId
                    AND JobCode = j.JobCode
                    AND EffectiveDate < j.EffectiveDate
            ORDER  BY EffectiveDate DESC       
       ) ca  

Answer 2

您可以使用 cross apply / top 1 连接外部查询中的派生表表达式，并一次选择所有相关列。

然后，您的查询可能如下所示：

SELECT ..., ISNULL(x, defaultValues.x)
FROM ...
  CROSS APPLY (SELECT TOP(1) x, y, z FROM ... WHERE ... ORDER BY ...) defaultValues

它将更加高效，因为查询中的联接较少（每个原始相关子查询都变成了外联接）。

您还可以使用 OUTER APPLY 来实现类似外部联接的功能。