本文介绍如何旋转 SQL Server 表。 假设您有一个名为 QTRSALES 表。 表按以下格式 (请注意有是没有行 1996 年的第四季度) 包含与数据进行列 YEAR、 季度和金额:
Year Quarter Amount ------------------------------- 1995 1 125,000.90 1995 2 136,000.75 1995 3 212,000.34 1995 4 328,000.82 1996 3 728,000.35 1996 2 422,000.13 1996 1 328,000.82
现在,假设您要旋转,以便您可以看到下面的格式中数据的表:
YEAR Q1 Q2 Q3 Q4 ------------------------------------------------------------------------- 1995 125,000.90 136,000.75 212,000.34 328,000.82 1996 328,000.82 422,000.13 728,000.35 0.00示例查询以旋转表
以下是将用于旋转表的查询:SELECT YEAR, Q1= ISNULL((SELECT AMOUNT FROM QTRSALES WHERE QUARTER = 1 AND YEAR = Q.YEAR),0), Q2= ISNULL((SELECT AMOUNT FROM QTRSALES WHERE QUARTER = 2 AND YEAR = Q.YEAR),0), Q3= ISNULL((SELECT AMOUNT FROM QTRSALES WHERE QUARTER = 3 AND YEAR = Q.YEAR),0), Q4= ISNULL((SELECT AMOUNT FROM QTRSALES WHERE QUARTER = 4 AND YEAR = Q.YEAR),0) FROM QTRSALES Q GROUP BY YEAR大表的查询
大的表此查询将更快:-------------------------------------------------------------------------------SELECT year=q.year, SUM(CASE quarter WHEN 1 THEN amount ELSE 0 END) as Q1, SUM(CASE quarter WHEN 2 THEN amount ELSE 0 END) as Q2, SUM(CASE quarter WHEN 3 THEN amount ELSE 0 END) as Q3, SUM(CASE quarter WHEN 4 THEN amount ELSE 0 END) as Q4 FROM qtrsales q GROUP BY year
case还有许多用法,例举一二如下
sql中有没有类似switch case的语句呢?
没有,用case when 来代替就行了.
例如,下面的语句显示中文年月
select getdate() as 日期,case month(getdate())
when 11 then '十一'
when 12 then '十二'
else substring('一二三四五六七八九十', month(getdate()),1)
end+'月' as 月份
=================================================
CASE 可能是 SQL 中被误用最多的关键字之一。虽然你可能以前用过这个关键字来创建字段,但是它还具有更多用法。例如,你可以在 WHERE 子句中使用 CASE。
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首先让我们看一下 CASE 的语法。在一般的 SELECT 中,其语法如下:
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SELECT <myColumnSpec> =
CASE
WHEN <A> THEN <somethingA>
WHEN <B> THEN <somethingB>
ELSE <somethingE>
END
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在上面的代码中需要用具体的参数代替尖括号中的内容。下面是一个简单的例子:
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USE pubs
GO
SELECT
Title,
'Price Range' =
CASE
WHEN price IS NULL THEN 'Unpriced'
WHEN price < 10 THEN 'Bargain'
WHEN price BETWEEN 10 and 20 THEN 'Average'
ELSE 'Gift to impress relatives'
END
FROM titles
ORDER BY price
GO
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这是 CASE 的典型用法,但是使用 CASE 其实可以做更多的事情。比方说下面的 GROUP BY 子句中的 CASE:
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SELECT 'Number of Titles', Count(*)
FROM titles
GROUP BY
CASE
WHEN price IS NULL THEN 'Unpriced'
WHEN price < 10 THEN 'Bargain'
WHEN price BETWEEN 10 and 20 THEN 'Average'
ELSE 'Gift to impress relatives'
END
GO
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你甚至还可以组合这些选项,添加一个 ORDER BY 子句,如下所示:
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USE pubs
GO
SELECT
CASE
WHEN price IS NULL THEN 'Unpriced'
WHEN price < 10 THEN 'Bargain'
WHEN price BETWEEN 10 and 20 THEN 'Average'
ELSE 'Gift to impress relatives'
END AS Range,
Title
FROM titles
GROUP BY
CASE
WHEN price IS NULL THEN 'Unpriced'
WHEN price < 10 THEN 'Bargain'
WHEN price BETWEEN 10 and 20 THEN 'Average'
ELSE 'Gift to impress relatives'
END,
Title
ORDER BY
CASE
WHEN price IS NULL THEN 'Unpriced'
WHEN price < 10 THEN 'Bargain'
WHEN price BETWEEN 10 and 20 THEN 'Average'
ELSE 'Gift to impress relatives'
END,
Title
GO
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注意,为了在 GROUP BY 块中使用 CASE,查询语句需要在 GROUP BY 块中重复 SELECT 块中的 CASE 块。
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除了选择自定义字段之外,在很多情况下 CASE 都非常有用。再深入一步,你还可以得到你以前认为不可能得到的分组排序结果集。