case

case 是一个控制流语句，用于 SQL 的条件判断语句，其作用与 if-else 语句相似。

case 语句至少包含一组 when…then… 语句，当满足 when 条件，则返回 then 结果，否则返回 else 后的表达式结果；若没有指定 else，则返回空值。case 语句多个条件的返回结果类型必须一致。case when 语句支持在分布式查询中使用。

语法

case
    when condition_1 then expression_1
    [when condition_2 then expression_2]
    ...
    [when condition_n then expression_n]
    [else expression_end]
end

从 2.00.10 版本开始，不再要求 when 和 then 必须在同一行。

例子

t = table(`st0`st1`st2`st4`st5`st6 as sym, 80 200 150 220 130 190 as vol)
select sym, case 
	when vol < 100 then -1 
	when vol < 200 then 0
	else 1
end as flag
from t


sym	flag
st0	-1
st1	1
st2	0
st4	1
st5	0
st6	0

t = table(2022.01.01 2022.01.01 2022.01.01 2022.01.01 2022.01.01 2022.01.01 as date, `a`a`b`b`a`b as id, 300 290 302 296 304 320 as val)
select date, case when t.id == `a then val end as `GroupA, case when t.id == `b then val end as `GroupB from t


date	GroupA	GroupB
2022.01.01	300
2022.01.01	290
2022.01.01		302
2022.01.01		296
2022.01.01	304
2022.01.01		320

select sum(val) as total from t group by case when t.val < 300 then 0 else 1 end as flag


flag	total
1	1,226
0	586

支持在 case 语句中使用分析函数

t = table([2023.06M,2023.07M,2023.08M,2023.07M,2023.07M,2023.06M,2023.08M,2023.06M,2023.08M] as month, ["A","A","B","B","C","C","A","B","C"] as flag, [1200,1500,1200,1300,1400,1400,1400,1000,1300] as val)
t


month	flag	val
2023.06M	A	1,200
2023.07M	A	1,500
2023.08M	B	1,200
2023.07M	B	1,300
2023.07M	C	1,400
2023.06M	C	1,400
2023.08M	A	1,400
2023.06M	B	1,000
2023.08M	C	1,300

当 flag 为 A 时，按月分组计算 val 的偏差；当 flag 不为 A 时，按月分组计算 val 的标准差。

select month,flag,(case when flag = "A"
then (val - avg(val)over(partition by month))
else std(val)over(partition by month) end) as value
from t


month	flag	value
2023.06M	A	0
2023.07M	A	100
2023.08M	B	100
2023.07M	B	100
2023.07M	C	100
2023.06M	C	200
2023.08M	A	100
2023.06M	B	200
2023.08M	C	100