Question

select HId, TestM, *
from TestDb
where Hid in ( 4, 5, 7, 9, 1, 132, 312)

如果我运行上面的.sql文件，默认情况下第一列将按升序排序。

我确实希望Hid列按照where子句中的值的顺序排列。我怎样才能做到这一点？

Hid | TestM | ...
_______________
4

5

7

9

1

132

312

Answer 1

您必须在ORDER BY子句中使用SELECT HId, TestM, * FROM TestDb WHEN Hid IN ( 4, 5, 7, 9, 1, 132, 312) ORDER BY CASE Hid WHEN 4 THEN 1 WHEN 5 THEN 2 WHEN 7 THEN 3 WHEN 9 THEN 4 WHEN 1 THEN 5 WHEN 132 THEN 6 WHEN 312 THEN 7 END表达式：

ORDER BY

另请注意，表格只是无序集合，因此如果您将<dependency> <groupId>javax.ws.rs</groupId> <artifactId>javax.ws.rs-api</artifactId> <version>2.0-m10</version> </dependency>子句保留，则无法保证结果集的顺序。

Answer 2

SQL没有隐式排序。如果没有声明ORDER BY，则结果可以按任何顺序返回，并且每次对相同数据执行相同查询时可能会返回不同的顺序。

常用的方法是使用CASE语句，但这可能变得笨重...

ORDER BY
    CASE HId WHEN   4 THEN 1
             WHEN   5 THEN 2
             WHEN   7 THEN 3
             WHEN   8 THEN 4
             WHEN   1 THEN 5
             WHEN 123 THEN 6
             WHEN 312 THEN 7 END

另一种方法是在一个表上加入JOIN，该表包含要过滤的值和要排序的列。它的笨拙，关系性更强，更容易扩展......

SELECT
    TestDB.HId, TestDB.TestM, TestDB.*
FROM
    TestDB
INNER JOIN
(
              SELECT   4 AS val, 1 AS ordinal
    UNION ALL SELECT   5 AS val, 2 AS ordinal
    UNION ALL SELECT   7 AS val, 3 AS ordinal
    UNION ALL SELECT   8 AS val, 4 AS ordinal
    UNION ALL SELECT   1 AS val, 5 AS ordinal
    UNION ALL SELECT 132 AS val, 6 AS ordinal
    UNION ALL SELECT 312 AS val, 7 AS ordinal
)
    filter
        ON filter.value = TestDB.HId
ORDER BY
    filter.ordinal

在sql查询中禁用顺序

2 个答案: