一、格式说明
格式说明 1
1 case 列名 2 when 条件值1 then 选项1 3 when 条件值2 then 选项2....... 4 else 默认值 end
格式说明2
1 case 2 when 列名= 条件值1 then 选择项1 3 when 列名=条件值2 then 选项2....... 4 else 默认值 end
比较: 两种格式,可以实现相同的功能。
简单Case函数的写法相对比较简洁,但是和Case搜索函数相比,功能方面会有些限制,比如写判断式。还有一个需要注意的问题,Case函数只返回第一个符合条件的值,剩下的Case部分将会被自动忽略
二、举例子
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,举个例子:
1 SELECT 2 Title, 3 'Price Range' = 4 CASE 5 WHEN price IS NULL THEN 'Unpriced' 6 WHEN price < 10 THEN 'Bargain' 7 WHEN price BETWEEN 10 and 20 THEN 'Average' 8 ELSE 'Gift to impress relatives' 9 END 10 FROM titles 11 ORDER BY price
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:
1 SELECT 'Number of Titles', Count(*) 2 FROM titles 3 GROUP BY 4 CASE 5 WHEN price IS NULL THEN 'Unpriced' 6 WHEN price < 10 THEN 'Bargain' 7 WHEN price BETWEEN 10 and 20 THEN 'Average' 8 ELSE 'Gift to impress relatives' 9 END
你甚至还可以组合这些选项,添加一个 ORDER BY 子句,如下所示:
1 SELECT 2 CASE 3 WHEN price IS NULL THEN 'Unpriced' 4 WHEN price < 10 THEN 'Bargain' 5 WHEN price BETWEEN 10 and 20 THEN 'Average' 6 ELSE 'Gift to impress relatives' 7 END AS Range, 8 Title 9 FROM titles 10 GROUP BY 11 CASE 12 WHEN price IS NULL THEN 'Unpriced' 13 WHEN price < 10 THEN 'Bargain' 14 WHEN price BETWEEN 10 and 20 THEN 'Average' 15 ELSE 'Gift to impress relatives' 16 END, 17 Title 18 ORDER BY 19 CASE 20 WHEN price IS NULL THEN 'Unpriced' 21 WHEN price < 10 THEN 'Bargain' 22 WHEN price BETWEEN 10 and 20 THEN 'Average' 23 ELSE 'Gift to impress relatives' 24 END, 25 Title