If variable is Empty or NULL in WHERE clause in SQL Server

If variable is Empty or NULL in WHERE clause in SQL Server

If @SearchType is Null or Empty  how to return all values from the database.

Sample Query:
SELECT
        P.[ProductId],
        P.[ProductName],
        P.[ProductPrice],
        P.[Type]
    FROM [Product] P
    -- if @Searchtype is not null then use the where clause

    WHERE p.[Type] = @SearchType

Just use

If @searchType is null means 'return the whole table' then use

WHERE p.[Type] = @SearchType OR @SearchType is NULL
If @searchType is an empty string means 'return the whole table' then use

WHERE p.[Type] = @SearchType OR @SearchType = ''
If @searchType is null or an empty string means 'return the whole table' then use

WHERE p.[Type] = @SearchType OR Coalesce(@SearchType,'') = ''

If the @SearchType is Null or Empty, it returns all the value from the database.

If the @SearchType contains any value, it returns only the condition satisfied data.