Replace this "switch" statement by "if" statements to increase readability.

Code Path:server/src/main/java/org/apache/iotdb/db/mpp/plan/analyze/ExpressionUtils.java

switch statements are useful when there are many different cases depending on the value of the same expression. For just one or two cases however, the code will be more readable with if statements.

打开 server/src/main/java/org/apache/iotdb/db/mpp/plan/analyze/ExpressionUtils.java，并检查 issue 所标识的 switch 语句。将其替换为针对现有 case 的等效 if 语句，然后验证该文件仍能编译且相关检查均通过。

由索引模型根据 Issue 内容生成。