95. Avoid Divide-by-Zero with NULLIF
Dividing by zero would normally be a problem, but in SQLite division by NULL yields NULL (no error). The idiom x / NULLIF(divisor, 0) converts a zero divisor into NULL so the whole expression becomes NULL instead of blowing up — then you can wrap it in COALESCE to show a default.
Task: for each campaign return id, clicks, impressions, and ctr — the click-through rate clicks / impressions as a ratio rounded to 3 decimals. When impressions is 0, ctr must be 0.0 (not an error and not NULL). Order by id.
Output columns, in order: id, clicks, impressions, ctr.
This is the result for the example data above. On Submit your query is graded against this example plus 4 hidden edge cases — 5test cases in all. A sloppy query that only fits the example won't pass.