Question

Okay, we have a database, with a not-so-good tables-columns layout, for a small news agency functioning online. Changing the layout is, currently, not possible; so, I’m focused on the queries.

Webpage requires a list of “guest” authors. Each list item should contain:

• Author’s image (Authors)
• Author’s name (Authors)
• Title of the last article he/she wrote (News)
• URI of that article (News)

In parentheses are the table names that info is stored in.

In “Authors” (40+ records), relevant columns are:

• Image
• Name
• Type (we’ll look for the value “Guest” here)
• ASCII (stores the “Name” value but without non-English characters and spaces)

The “News” (28k+ records) table is, basically, a pool where all the news and articles are pouring in. Columns of interest would be:

• Id (primary key: the higher, the more recent)
• Category
• Title
• URI

Now, “Category” has a whole bunch of values; however, if the particular record is an article, this column holds the “ASCII” of “Authors” (rather than its “Id”, sadly).

There was a PHP code, “while”ing queries for each author. I wanted to replace that when I saw it. So, I thought, “Hmm… How to fetch these in one go?” and came up with this:

-- Aliases are prefixed with 't' for tables and 'c' for columns.
SELECT
  tAu.Image, tAu.Name, tNw.Title, tNw.URI
FROM ( -- tAu & tNw

    SELECT * FROM ( -- tRc & Authors

        SELECT
          MAX(Id) cId, Category cCt -- Max Id for most recent
        FROM
          News
        GROUP BY
          cCt

    ) tRc -- table of categories with their most recent id's
    INNER JOIN
      Authors
    ON
      tRc.cCt = Authors.ASCII
    WHERE
      Authors.Type = 'Guest'

) tAu -- table of authors with their most recent id's
INNER JOIN
  News tNw
ON
  tAu.cId = tNw.Id -- merging authors with their latest article info

Currently, this query takes around 0.0364 seconds. May not be that bad; but, I am curious if this can be made any better (as this query SELECTs twice from News).

Answer 1

I don’t know if it is faster or not, but cleaned up your query a bit, which may help the optimizer:

SELECT
  Authors.Image,
  Authors.Name,
  News.Title,
  News.URI

FROM (
    SELECT
      MAX(Id) TopNewsId,
      Category
    FROM News
    GROUP BY Category
) TopNewsPerCats

INNER JOIN News     ON News.Id       = TopNewsPerCats.TopNewsId

INNER JOIN Authors  ON Authors.ASCII = TopNewsPerCats.Category
                   AND Authors.Type  = 'Guest'

News table should have an index starting with ( Category, Id) to speed up the internal SELECT GROUP BY.