如何优化 MSSQL CASE WHEN 查询

Question

这是我的代码示例：

SET @variable_out = 
'Report: '
+ CASE WHEN (SELECT name FROM person WITH(NOLOCK) WHERE person_id = @person_id) != ''
  THEN 'Name: ' + (SELECT name FROM person WITH(NOLOCK) WHERE person_id = @person_id) + CHAR(13)+CHAR(10)
  ELSE 'Name: not found' + CHAR(13)+CHAR(10)
END
+ CASE WHEN (SELECT home_phone FROM person WITH(NOLOCK) WHERE person_id = @person_id) != ''
  THEN 'Phone #: ' + (SELECT home_phone FROM person WITH(NOLOCK) WHERE person_id = @person_id) + CHAR(13)+CHAR(10)
  ELSE 'Phone #: not found' + CHAR(13)+CHAR(10)
END

等等……

如您所见，我对正在构建的变量的每个 CASE WHENE... 重复执行两次选择，我希望将其折叠为每行仅选择一次。

我知道的唯一解决方案是为 CASE WHEN... 创建一个唯一变量，预先运行所有选择，然后如果变量不为空，将它们连接到 @variable_out。

有没有更聪明的方法来做到这一点？

Answer 1

DECLARE  @name the_same_datatype_as_name_field_from_person_table --Ex. VARCHAR(100)
    ,@home_phone the_same_datatype_as_homephone_field_from_person_table; --Ex. VARCHAR(15)

SELECT @name  = NULLIF(p.name,''), @home_phone = NULLIF(p.home_phone,'')
FROM person p --WITH(NOLOCK)
WHERE p.person_id = @person_id;

SET @variable_out = 
'Report: ' 
+ ISNULL('Name: ' + @name, 'Name: not found')
+ CHAR(13)+CHAR(10)
+ ISNULL('Phone #: ' + @home_phone, 'Phone #: not found')
+ CHAR(13)+CHAR(10);

注意：

1. 或者，您可以为 @name 和 @home_phone 变量使用与 @variable_out 变量相同的数据类型（例如 VARCHAR）。

2. NOLOCK pros & cons.