Case用法
2013-07-21 15:59
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SELECT <myColumnSpec> = CASE WHEN <A> THEN <somethingA> WHEN <B> THEN <somethingB> ELSE <somethingE> END 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: 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 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: 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 你甚至还可以组合这些选项,添加一个 ORDER BY 子句,如下所示: 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 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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