Question

我有以下sql查询

GO
SELECT @StartDate= table1.StartDate, @EndDate= table2.EndDate from table1 join table2

SELECT @StartDate= table2.StartDate, @EndDate= table3.EndDate from table2 join table3

SELECT @StartDate= table3.StartDate, @EndDate= table4.EndDate from table3 join table4

基本上，如果最后一个查询不为null，那么这将是@StartDate和@EndDate的最终值。对此最好的方法是什么，我正在考虑使用ISNULL，COALESCE或CASE WHEN

Answer 1

我认为不需要3个查询来获得所需的结果。 with open("/Users/jj/Desktop/Courtney_fan.txt") as f: sd = f.read() sd_list = eval(sd)函数仅评估一次。 ISNULL表达式可以多次评估。

COALESCE

Answer 2

DECLARE @DataSource TABLE
(
    [RecordID] TINYINT PRIMARY KEY
);

DECLARE @DataSourceDates01 TABLE
(
    [RecordID] TINYINT PRIMARY KEY
   ,[StartDate] DATETIME2
   ,[EndDate] DATETIME2
);

DECLARE @DataSourceDates02 TABLE
(
    [RecordID] TINYINT PRIMARY KEY
   ,[StartDate] DATETIME2
   ,[EndDate] DATETIME2
);


DECLARE @DataSourceDates03 TABLE
(
    [RecordID] TINYINT PRIMARY KEY
   ,[StartDate] DATETIME2
   ,[EndDate] DATETIME2
);

INSERT INTO @DataSource ([RecordID])
VALUES (1), (2), (3);

INSERT INTO @DataSourceDates01 ([RecordID], [StartDate], [EndDate])
VALUES (1, GETUTCDATE(), GETUTCDATE());

INSERT INTO @DataSourceDates02 ([RecordID], [StartDate], [EndDate])
VALUES (1, GETUTCDATE(), GETUTCDATE())
      ,(2, GETUTCDATE(), GETUTCDATE());

INSERT INTO @DataSourceDates03 ([RecordID], [StartDate], [EndDate])
VALUES (1, GETUTCDATE(), GETUTCDATE())
      ,(2, GETUTCDATE(), GETUTCDATE())
      ,(3, GETUTCDATE(), GETUTCDATE());

SELECT DS.[RecordID]
      ,COALESCE(DS1.[StartDate], DS2.[StartDate], DS3.[StartDate]) AS [StartDate]
      ,COALESCE(DS1.[EndDate], DS2.[EndDate], DS3.[EndDate]) AS [EndDate]
FROM @DataSource DS
LEFT JOIN @DataSourceDates01 DS1
    ON DS.[RecordID] = DS1.[RecordID]
LEFT JOIN @DataSourceDates02 DS2
    ON DS.[RecordID] = DS2.[RecordID]
LEFT JOIN @DataSourceDates03 DS3
    ON DS.[RecordID] = DS3.[RecordID];

使用COALESCE，ISNULL或CASE时重构

2 个答案: