Question

A simple quiz:

Probably many guys know this before,

In my app there is a query in which Im using concat in where condition like this,

v_book_id and v_genre_id are 2 variables in my procedure.

SELECT link_id 
FROM link
WHERE concat(book_id,genre_id) = concat(v_book_id,v_genre_id);

Now, I know there is a catch/bug in this, which will occur only twice in your lifetime. Can you tell me what is it?

I found this out yesterday and thought I should make a noise about all others practicing this.

Thanks.

Answer 1

Let’s have a look

WHERE concat(book_id,genre_id) = concat(v_book_id,v_genre_id);

as opposed to

WHERE book_id = v_book_id AND genre_id = v_genre_id;

There. The second solution is

• faster (optimal index usage)
• easier to write (less code)
• easier to read (what on earth was the author thinking to concatenate numbers???)

• more correct (as Alnitak also stated in the question’s comments). check out this sample data:

  book_id | genre_id
  1       | 12
  11      |  2
  

  Now add (or concat) v_book_id = 1 and v_genre_id = 12 and see how you’ll get funny results with your concat() query

Note, some databases (including MySQL) allow operations on tuples, which may be what the clever author of the above really intended to do:

WHERE (book_id, genre_id) = (v_book_id, v_genre_id);

A working example of such a tuple predicate:

SELECT * FROM (
  SELECT 1 x, 2 y FROM DUAL UNION ALL
  SELECT 1 x, 3 y FROM DUAL UNION ALL
  SELECT 1 x, 2 y FROM DUAL 
) a
WHERE (x, y) = (1, 2)

Note, some databases will need extra parentheses around the right-hand side tuple : ((1, 2))