首先，CASE 语句必须是表达式的一部分，而不是表达式本身.

First off, the CASE statement must be part of the expression, not the expression itself.

换句话说，你可以:

WHERE co.DTEntered = CASE 
                          WHEN LEN('blah') = 0 
                               THEN co.DTEntered 
                          ELSE '2011-01-01' 
                     END 

但它不会像你写的那样工作，例如:

But it won't work the way you have written them eg:

WHERE 
    CASE LEN('TestPerson')
        WHEN 0 THEN co.personentered  = co.personentered
   ELSE co.personentered LIKE '%TestPerson'
    END 

使用这样的组合 OR 语句可能会更好:

You may have better luck using combined OR statements like this:

WHERE (
        (LEN('TestPerson') = 0 
             AND co.personentered = co.personentered
        ) 
        OR 
        (LEN('TestPerson') <> 0 
             AND co.personentered LIKE '%TestPerson')
      )

虽然，无论哪种方式，我都不确定您会获得多棒的查询计划.WHERE 子句中的这些类型的恶作剧通常会阻止查询优化器使用索引.

Although, either way I'm not sure how great of a query plan you'll get. These types of shenanigans in a WHERE clause will often prevent the query optimizer from utilizing indexes.