IF 话语为什么不能放到 INSERT INTO 语句内

IF 语句为什么不能放到 INSERT INTO 语句内？

SQL code

ALTER PROCEDURE [dbo].[sp_ProductGetList]--------------------------------------目的：常规返回产品列表(以分页形式)----维护日志----维护人                维护时间                描述-------------------        -------------------        --------------------------xuyiwei                2012-7-24                创建--------------------------------------分类Id@Category INT,--搜索关键字@Key NVARCHAR(50),--当前页码@PageNumber INT,--每页多少@PageSize INT,--返回总共多少行@HowMany INT OUTPUTAS-- 声明新的TABLE变量DECLARE @Product TABLE( RowNumber INT, Id INT, Nick NVARCHAR(50), NumIid bigint, TrackIiid NVARCHAR(50), ContactPhone NVARCHAR(50),CategoryId INT, CreateTime DATETIME, ModifiedTime DATETIME, Sorting INT, EffectiveIntegral INT, ShareIntegral INT )-- 用完整的产品列表填充该TABLE变量INSERT INTO @ProductSELECT ROW_NUMBER() OVER (ORDER BY Product.Id),       Id, Nick,NumIid, TrackIiid,ContactPhone,CategoryId,CreateTime,ModifiedTime,Sorting,EffectiveIntegral,ShareIntegralFROM Product--WHERE Id > 0想在这里判断参数不空的话就查询相关字段，可好象放这里不行--IF @Category > 0--AND CategoryId = @Category --IF @Key <> ''--AND Nick = @Key ORDER BY Id DESC-- 使用OUTPUT变量返回总共多少行SELECT @HowMany = COUNT(Id) FROM @Product-- 获取请求的商品页面SELECT Id, Nick,NumIid, TrackIiid,ContactPhone,CategoryId,CreateTime,ModifiedTime,Sorting,EffectiveIntegral,ShareIntegralFROM @ProductWHERE RowNumber > (@PageNumber - 1) * @PageSize  AND RowNumber <= @PageNumber * @PageSize  

SQL code

ALTER PROCEDURE [dbo].[sp_ProductGetList]--------------------------------------目的：常规返回产品列表(以分页形式)----维护日志----维护人                维护时间                描述-------------------        -------------------        --------------------------xuyiwei                2012-7-24                创建--------------------------------------分类Id@Category INT,--搜索关键字@Key NVARCHAR(50),--当前页码@PageNumber INT,--每页多少@PageSize INT,--返回总共多少行@HowMany INT OUTPUTAS-- 声明新的TABLE变量DECLARE @Product TABLE( RowNumber INT, Id INT, Nick NVARCHAR(50), NumIid bigint, TrackIiid NVARCHAR(50), ContactPhone NVARCHAR(50),CategoryId INT, CreateTime DATETIME, ModifiedTime DATETIME, Sorting INT, EffectiveIntegral INT, ShareIntegral INT )-- 用完整的产品列表填充该TABLE变量INSERT INTO @ProductSELECT ROW_NUMBER() OVER (ORDER BY Product.Id),       Id, Nick,NumIid, TrackIiid,ContactPhone,CategoryId,CreateTime,ModifiedTime,Sorting,EffectiveIntegral,ShareIntegralFROM ProductWHERE Id > 0想在这里判断参数不空的话就查询相关字段，可好象放这里不行AND @Category > 0AND CategoryId = @Category AND @Key <> ''AND Nick = @Key ORDER BY Id DESC-- 使用OUTPUT变量返回总共多少行SELECT @HowMany = COUNT(Id) FROM @Product-- 获取请求的商品页面SELECT Id, Nick,NumIid, TrackIiid,ContactPhone,CategoryId,CreateTime,ModifiedTime,Sorting,EffectiveIntegral,ShareIntegralFROM @ProductWHERE RowNumber > (@PageNumber - 1) * @PageSize  AND RowNumber <= @PageNumber * @PageSizewhere后面怎么可以用IF语句，楼主仔细分析一下自己的思路，到底应该是先判断在执行对应操作还是把数据筛选出来了，在判断条件看该不该查询某些东西。从逻辑上分析一下
[解决办法]
where中可以大致这么写：

WHERE Id > 0
and (@Category = 0 or CategoryId = @Category) -- 假如@Category = 0， CategoryId = @Category这个条件就会忽略
and (@Key = '' or Nick = @Key) --同上
ORDER BY Id DESC