PL/SQL中if语句中的标量子查询 [英] scalar subquery in if statement Condition in PL/SQL

问题描述

我有一个类似于以下内容的If语句块，但由于错误而失败- PLS-00103:预期以下情况之一时遇到符号"SELECT".

I have an If Statement block similar to the below which is failing with the error - PLS-00103: Encountered the symbol "SELECT" when expecting one of the following....

Begin
    If (select count(*) from Table1) > 0 then
        dbms_output.put_line('Test');
    end if;
end;

我有类似的Case语句，效果很好

I have similar Case statement which works fine

select 
case 
    when (select count(*) from Table1) > 0
        then 2
    else
        1
end
from dual

根据我在Oracle文档中所读的内容，如果以及何时支持布尔表达式，则可以了解If条件中是否支持子查询.

From what i have read in Oracle Documentation the if and when support a Boolean Expression, any ideas whether Subqueries are supported in If Conditions.

注意:语句已被简化，我并没有真正获得整个表的数量，所以请没有优化建议

Note: The Statements have been simplified, i am not really going to get the count of the entire table, so no optimization suggestions please

推荐答案

不，您不能以所需的方式使用SELECT.

No, you can't use a SELECT in the way you want.

在使用CASE的示例中，您没有使用CASE语句"，而是使用了CASE 表达式，它恰好嵌入在SQL语句中.在这种情况下，可以使用子查询，因为它在SQL语句(而不是过程语句)的上下文中.您将无法在程序化CASE语句中使用像这样的子查询.

In your example using CASE, you are not using a CASE "statement" -- you are using a CASE expression, which happens to be embedded within a SQL statement. You can use a subquery in that case because it's within the context of a SQL statement, not a procedural statement. You wouldn't be able to use a subquery like this in a procedural CASE statement.

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