SQL 查询在 Azure SQL 数据仓库 (Synapse) 中不起作用，为什么不呢？

如何解决SQL 查询在 Azure SQL 数据仓库 (Synapse) 中不起作用，为什么不呢？

我有一个可在本地 SQL 数据库中运行的 SQL 查询，但是当我尝试在 Azure SQL 数据仓库上执行它时，出现错误。 有谁知道编写此 SQL 查询的另一种方法，以便它可以在 Azure DW 中使用？

SQL 查询是：

SELECT MAX(Dates) MostRecentDate FROM (VALUES('2020-01-01'),('2021-01-01')) AS t(Dates)

Azure DW 错误是：

消息 103010，级别 16，状态 1，第 1 行 第 1 行解析错误，第 39 列：“(”附近的语法不正确。

这里是完整的SQL查询（真实的表名已被删除）

SELECT t1.reference,dd.closest_date,d1.date_one,d2.date_two,d3.date_three
FROM dbo.table1 AS t1
LEFT JOIN dbo.table2 AS t2 ON t1.table2_id = t2.id
LEFT JOIN dbo.table3 AS t3 ON t1.table3_id = t3.id
LEFT JOIN dbo.table4 AS t4 ON t3.table4_id = t4.id
LEFT JOIN dbo.table5 AS t5 ON t4.table5_id = t5.id
LEFT JOIN dbo.table6 AS t6 ON t5.table6_id = t6.id
OUTER APPLY (SELECT CASE WHEN t3.outcome IS NULL THEN '5000-01-01' ELSE ISNULL(t3.outcome_date,'5000-01-01') END AS date_one) AS d1
OUTER APPLY (SELECT ISNULL(t2.outcome_date,'5000-01-01') AS date_two) AS d2
OUTER APPLY (SELECT ISNULL(t6.outcome_date,'5000-01-01') AS date_three) AS d3
/*the below works in normal SQL,but doesn't work in Azure SQL!!!*/
--  OUTER APPLY (SELECT MIN(Dates) closest_date FROM (VALUES(d1.date_one),(d2.date_two),(d3.date_three)) AS t(Dates)) AS dd

解决方法

Table Value Constructor 很好，但根据文档，Azure Synapse Analytics 不完全支持，即链接文档顶部的适用于列表中没有 Synapse。根据 this example，单行支持 VALUES，但对于您的示例，最简单的解决方法是简单地将其重写为简单的 UNION ALL 语句，例如

SELECT t1.reference,dd.closest_date,d1.date_one,d2.date_two,d3.date_three
FROM dbo.table1 AS t1
    LEFT JOIN dbo.table2 AS t2 ON t1.table2_id = t2.id
    LEFT JOIN dbo.table3 AS t3 ON t1.table3_id = t3.id
    LEFT JOIN dbo.table4 AS t4 ON t3.table4_id = t4.id
    LEFT JOIN dbo.table5 AS t5 ON t4.table5_id = t5.id
    LEFT JOIN dbo.table6 AS t6 ON t5.table6_id = t6.id
    OUTER APPLY (SELECT CASE WHEN t3.outcome IS NULL THEN '5000-01-01' ELSE ISNULL(t3.outcome_date,'5000-01-01') END AS date_one) AS d1
    OUTER APPLY (SELECT ISNULL(t2.outcome_date,'5000-01-01') AS date_two) AS d2
    OUTER APPLY (SELECT ISNULL(t6.outcome_date,'5000-01-01') AS date_three) AS d3

/*the below works in normal SQL,but doesn't work in Azure Synapse*/
    --OUTER APPLY (SELECT MIN(Dates) closest_date FROM (VALUES(d1.date_one),(d2.date_two),(d3.date_three)) AS t(Dates)) AS dd
    OUTER APPLY (SELECT MIN(Dates) closest_date FROM ( SELECT d1.date_one UNION ALL SELECT d2.date_two UNION ALL SELECT d3.date_three) AS t(Dates)) AS dd

这真的只是语法糖。它们都是在一天结束时设置的操作。

,

你可以试试这个吗

 select max(t.dates)from
  (
      select '2020-01-01'as dates
        union all
      select '2021-01-01'
  )as t