Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>The comma separated list of tables in your FROM clause functions like an inner join, so your query is selecting all of the rows in the tagmaps table and the tags table that have the same tag ID, and of those rows, all of the rows from the books table and the tagmaps table that have the same book ID. The HAVING clause then requires that two rows be returned from that result set with the same book ID. There can only be one row in the books table with any given book ID (assuming book ID is the primary key of the books table), so this condition is never met.</p> <p>What you want is a join without the books table. You are looking for the same book ID appearing twice in the results of the OR clauses (I believe), so you don't want to join the books table with those results because that will ensure you can never have the same book ID in the results more than once.</p> <p>Edit: conceptually, you are essentially combining two different things. You are looking for tags and tagmaps for the same book, and you are also getting the book info from each of those books. So you are actually pulling duplicate other_info data for every instance of the same book ID in the tagmaps table, and then using the distinct clause to reduce that duplicate data down to one row, because all you want is the book ID and other_info. I would consider using two queries or a subquery to do this. There may be other [better] ways as well. I'd have to play around with it to figure it out.</p> <p>For starters, try</p> <pre><code>SELECT DISTINCT tm.book_id, b.other_info FROM tagmap tm inner join tags t on tm.tag_id = t.tag_id left join books b on tm.book_id = b.book_id HAVING count(tm.book_id) = 2 </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POMySQL search for item with all tags
      singulars
      1. PTQuestion
    1. PTAnswer
    1. UStaz
    1. UStaz
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POMySQL search for item with all tags
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    1. COI think I understand. However, after removing the "books b" join, replacing every b.book_id with tm.book_id, and removing the last AND clause, I still do not get a result. Weird thing though: I tried with two different tags that also belong to an item, and it worked fine for that one, even with my incorrect query. Other items/tags still don't work though, what's up with that?
      singulars
      1. PO
      1. USFang
    2. COI found a solution to my problem (which I'll add to the question in a minute), but I will definitely look into queries similar to the one you suggested. Guess there are multiple roads that lead to Rome, eh. Thank you so much for your help!
      singulars
      1. PO
      1. USFang
    3. CO@Fang No problem. Sorry I didn't have time to get back to you sooner. The `GROUP BY` should also work for your purpose, although you still might be pulling more data than you need. Regarding still getting no results, I'd recommend making sure you are explicitly joining tables with a JOIN clause instead of using a comma list and checking that you're using the correct ID fields. Based on the info you gave & without actually testing the SQL, I think it should work. You pull all tagmaps & tags that have the same tagID, and then you pull all books that have the same bookID as each tagID.
      singulars
      1. PO
      1. UStaz
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload