SQL 如何根据 CASE 执行不同的 SELECT 语句
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How do I execute different SELECT statements based on a CASE
提问by surbhit4u
I am facing a problem in executing queries with CASE statement. Based on my condition,(for eg. length), I want to execute different SQL statement.
我在使用 CASE 语句执行查询时遇到问题。根据我的条件（例如长度），我想执行不同的 SQL 语句。
Problematic sample query is as follows:
有问题的示例查询如下：
select case
when char_length('19480821') = 8
then select count(1) from Patient
when char_length('19480821')=10
then select count(1) from Doctor
end
Exception:
例外：
[Error] Script lines: 1-5 --------------------------
Incorrect syntax near the keyword 'select'.
Msg: 156, Level: 15, State: 2
Server: sunsrv4z7, Line: 2
[错误] 脚本行：1-5 --------------------------
关键字“select”附近的语法不正确。
消息：156，级别：15，状态：2
服务器：sunsrv4z7，行：2
I am not able to correct the syntax. I am getting the string for char_length as input from the user. How can I fire queries based on certain condition? Is CASE the right choice ? Or do I have to use any other thing.
我无法更正语法。我从用户那里获取 char_length 的字符串作为输入。如何根据特定条件触发查询？CASE 是正确的选择吗？或者我必须使用任何其他东西。
回答by Pranay Rana
Just put opening and closing bracket around select statement resolve you problem
只需在 select 语句周围放置左括号和右括号即可解决您的问题
select
case when
char_length('19480821')=8 then
(select count(1) from Patient )
when
char_length('19480821')=10 then
(select count(1) from Doctor )
end
回答by Phlamingo
Please do note that it is not a case STATEMENT, it is a case EXPRESSION. By enclosing the queries in parentheses, you are converting them (syntactically) to values.
请注意，这不是一个 case STATEMENT，而是一个 case EXPRESSION。通过将查询括在括号中，您将它们（在语法上）转换为值。
This is similar in principle to a subquery, such as " select name from Doctor where salary = (select max(salary) from Doctor)"
这在原理上类似于子查询，例如“ select name from Doctor wheresalary = (select max(salary) from Doctor)”
回答by VoodooChild
select
case when char_length('19480821')=8 then (select count(1) from Patient)
when char_length('19480821')=10 then (select count(1) from Doctor)
end
The problem is that you are missing opening and closing brackets in your nested 'Select' statements :)
问题是您在嵌套的“选择”语句中缺少左括号和右括号:)
回答by Terrence Harry Holmes
select
case when
LEN('1948082100')=8 then
(select 'HELLO' )
when
LEN('194808210')=10 then
(select 'GOODBYE')
end
Change the values to test results.
将值更改为测试结果。
