[英]STRING_AGG not behaving as expected
我有以下查询:
WITH cteCountryLanguageMapping AS (
SELECT * FROM (
VALUES
('Spain', 'English'),
('Spain', 'Spanish'),
('Sweden', 'English'),
('Switzerland', 'English'),
('Switzerland', 'French'),
('Switzerland', 'German'),
('Switzerland', 'Italian')
) x ([Country], [Language])
)
SELECT
[Country],
CASE COUNT([Language])
WHEN 1 THEN MAX([Language])
WHEN 2 THEN STRING_AGG([Language], ' and ')
ELSE STRING_AGG([Language], ', ')
END AS [Languages],
COUNT([Language]) AS [LanguageCount]
FROM cteCountryLanguageMapping
GROUP BY [Country]
我希望瑞士的 Languages 列中的值以逗号分隔,即:
| Country | Languages | LanguageCount
--+-------------+-------------------------------------------+--------------
1 | Spain | Spanish and English | 2
2 | Sweden | English | 1
3 | Switzerland | French, German, Italian, English | 4
相反,我得到了以下输出(4 个值由and
分隔):
| Country | Languages | LanguageCount
--+-------------+-------------------------------------------+--------------
1 | Spain | Spanish and English | 2
2 | Sweden | English | 1
3 | Switzerland | French and German and Italian and English | 4
我错过了什么?
这是另一个例子:
SELECT y, STRING_AGG(z, '+') AS STRING_AGG_PLUS, STRING_AGG(z, '-') AS STRING_AGG_MINUS
FROM (
VALUES
(1, 'a'),
(1, 'b')
) x (y, z)
GROUP by y
| y | STRING_AGG_PLUS | STRING_AGG_MINUS
--+---+-----------------+-----------------
1 | 1 | a+b | a+b
这是 SQL Server 中的错误吗?
是的,这是一个错误 (tm),存在于所有版本的 SQL Server 2017(截至撰写时)。 它已在 Azure SQL Server 和 2019 RC1 中修复。 具体来说,优化器中执行公共子表达式消除的部分(确保我们不计算不必要的表达式)错误地认为所有形式为STRING_AGG(x, <separator>)
表达式只要x
匹配,无论如何都相同<separator>
是,并将它们与查询中的第一个计算表达式统一起来。
一种解决方法是通过对其执行某种(近)身份转换来确保x
不匹配。 由于我们正在处理字符串,因此连接一个空字符串将执行以下操作:
SELECT y, STRING_AGG(z, '+') AS STRING_AGG_PLUS, STRING_AGG('' + z, '-') AS STRING_AGG_MINUS
FROM (
VALUES
(1, 'a'),
(1, 'b')
) x (y, z)
GROUP by y
不要重复自己*。 您正在通过使用MAX(...)
, LIST_AGG(...', ')
和LIST_AGG(...' and ')
来重复自己。 您可以像这样简单地重写查询,并可能得到更好的计划:
WITH cteCountryLanguageMapping AS (
SELECT * FROM (
VALUES
('Spain', 'English'),
('Spain', 'Spanish'),
('Sweden', 'English'),
('Switzerland', 'English'),
('Switzerland', 'French'),
('Switzerland', 'German'),
('Switzerland', 'Italian')
) x (Country, Language)
), results AS (
SELECT
Country,
COUNT(Language) AS LanguageCount,
STRING_AGG(Language, ', ') AS Languages
FROM cteCountryLanguageMapping
GROUP BY Country
)
SELECT Country, LanguageCount, CASE LanguageCount
WHEN 2 THEN REPLACE(Languages, ', ', ' and ')
ELSE Languages
END AS Languages_Fixed
FROM results
结果:
| Country | LanguageCount | Languages_Fixed |
|-------------|---------------|----------------------------------|
| Spain | 2 | Spanish and English |
| Sweden | 1 | English |
| Switzerland | 4 | French, German, Italian, English |
*我也不想通过说这是一个错误来重复别人。
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