高手帮忙解答下这个存储过程，有些语句看不懂,该怎么处理

高手帮忙解答下这个存储过程，有些语句看不懂

SQL code

CREATE proc [dbo].[spUpdClerkPwd] @ClerkId varchar(20),        --被重置操作员号             @OPassWord varchar(20),    --旧密码 @NPassWord varchar(20),    --新密码 @OperateId varchar(20)        --操作者员号（谁操作的）     asbeginset nocount ondeclare @field intdeclare @err intset @err=0declare @rowClerk intdeclare @rowInfo intdeclare @rowRelation intselect @rowClerk=COUNT(1) from Clerk where ClerkId = @ClerkId--select @rowClerk as rowClerk , @rowInfo as rowInfo, @rowRelation as rowRelationdeclare @Clerkrowcount intdeclare @oldClerk table(    Id int,    [ClerkId] [varchar](20)  NULL,    [PassWord] [varchar](20)  NULL)begin tranbegin try--这段下面的看不懂，请解释下 update a set a.PassWord=(case when isnull(@OPassWord, '~NULL~')=ISNULL(@NPassWord, '~NULL~') then a.PassWord else @NPassWord end),UpdateDT=GETDATE()output deleted.Id,deleted.ClerkId,deleted.PassWord  into @oldClerkfrom Clerk as a  where  a.ClerkId =@ClerkIdand isnull(a.PassWord, '~NULL~')=isnull(@OPassWord, '~NULL~')and a.Flag=0 set @Clerkrowcount=@@ROWCOUNT --PRINT  @ClerkRoleRelationrowcountif @Clerkrowcount=@rowClerk  commitelsebeginrollback if @rowClerk=(select COUNT(1) from @oldClerk as a  where a.ClerkId = @ClerkId  )set @err=50010elseset @err=50013goto exit_bkend

[解决办法]
case when isnull(@OPassWord, '~NULL~')=ISNULL(@NPassWord, '~NULL~')

两个变量相等，或者两个变量都为null
[解决办法]
update a set 
a.PassWord=(case when isnull(@OPassWord, '~NULL~')=ISNULL(@NPassWord, '~NULL~') then a.PassWord else @NPassWord end)

@OPassWord和@NPassWord都为空或者都不为空且相等......
[解决办法]
是输出这几个列，并且赋值到@oldClerk表里面
[解决办法]

SQL code

--这段下面的看不懂，请解释下 --更新这个表a 设置Password字段等于@NPassWord 变量--如果新密码和旧密码相等，更新PassWord等于PassWord，和更新1=1是一样，就是没变化UPDATE  aSET     a.PassWord = ( CASE WHEN ISNULL(@OPassWord, '~NULL~') = ISNULL(@NPassWord,                                                              '~NULL~')                            THEN a.PassWord                            ELSE @NPassWord                       END ) ,        UpdateDT = GETDATE()        --更新的同时用output子句输入被更新掉的结果进入到@oldClerk表变量OUTPUT  deleted.Id ,        deleted.ClerkId ,        deleted.PassWord        INTO @oldClerkFROM    Clerk AS a--这就是个where条件WHERE   a.ClerkId = @ClerkId        AND ISNULL(a.PassWord, '~NULL~') = ISNULL(@OPassWord, '~NULL~')        AND a.Flag = 0 SET @Clerkrowcount = @@ROWCOUNT
[解决办法]
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引用:
SQL code

--这段下面的看不懂，请解释下
--更新这个表a 设置Password字段等于@NPassWord 变量
--如果新密码和旧密码相等，更新PassWord等于PassWord，和更新1=1是一样，就是没变化
UPDATE  a
SET     a.PassWord = ( CASE WHEN ISNULL(@OPas……