The simplest and best way to get yesterday's date is:

subdate(current_date, 1)

Your query would be:

    url as LINK,
    count(*) as timesExisted,
    sum(DateVisited between UNIX_TIMESTAMP(subdate(current_date, 1)) and
        UNIX_TIMESTAMP(current_date)) as timesVisitedYesterday
FROM mytable

For the curious, the reason that sum(condition) gives you the count of rows that satisfy the condition, which would otherwise require a cumbersome and wordy case statement, is that in mysql boolean values are 1 for true and 0 for false, so summing a condition effectively counts how many times it's true. Using this pattern can neaten up your SQL code.