Question

i have a procedure in which the below condition is to be written in a WHERE clause. How do I do that.

itemid is a parameter which can be null.

if itemid is available then add it to my where clause,else do nothing

Answer 1

Some people use this technique

... WHERE @itemid IS NULL OR tbl.itemid = @itemid

It guarantees though that you will never get an index seek on the itemid column.

A better approach if the table is at all big is to split the query up into 2 separate cases

IF(@itemid IS NULL)
  SELECT foo FROM bar
ELSE
  SELECT foo FROM bar WHERE itemid = @itemid

If the number of combinations is too large you can consider dynamic SQL. Be sure you understand SQL injection first.

Ref: Dynamic Search Conditions in T-SQL