Comparing datetime value of columns from different tables

Question

I have my sql query like this

if not exists(select RowId from dbo.Cache where StringSearched = @FirstName and colName = 'FirstName')
begin
   --some code here 
 end

The purpose of above if statement is not to execute the piece of code inside of it if value of StringSearched is already present in Cache table which means it has been looked up before and so no need to make calculations again. The code inside of if statement if executed returns row number of rows from Table B and those are then inserted into Cache table to continue maintaining the cache. anyway .I need the records to be picked from Cache only if ModifiedAt column of Cache table is latest than ModifiedAt column of rows of Table B .

Note: I understand that I may need to use a subquery in where clause but in where clause itself, I need to check ModifiedAt column of Table B only for RowId' s returned by Outer select query .

How can I proceed without making it much complex ?

Answer 1

You can use the subquery in the current query along with the Where clause.You didn't specified what are the columns to know for figure out which rows to get value so I assumed your tableB also has StringSearched and colName to get max(ModifiedAt) for that string vlaue.

IF NOT EXISTS (SELECT * from dbo.Cache as c WHERE StringSearched = @FirstName 
                                             AND colName = 'FirstName' 
                                             AND ModifiedAt  > (Select MAX(ModifiedAt) FROM tableB as tabB WHERE tabB.RowID = c.RowID ))
 BEGIN
    --your query
 END